Sabtu, 29 Oktober 2011
Hybridization
Hybridization defined: To produce or cause to produce hybrids; crossbreed.
If the definition isn't working for you I'll explain it. A Hybrid is a finch (though this applies to all animals) that is produced from the pairing of two different species of finch. A rather common example is an Owl finch and Zebra finch pair. When bred together they produce an Owl/Zebra Finch Hybrid. The hybrid will exhibit traits of both the Zebra finch and the Owl finch. Often they literally look like a blend of the two species.
Hybrids do occur in the wild as well in aviary set-ups. There are rather significant flaws with hybrids, which is why I am against the idea of intentionally producing them.
Flaws:
Let's go to the wilderness first. While Hybrids do occur in nature they almost never develop into a viable species. The first major problem is that not all Hybrids work. Some species pairings simply will not produce young. They are far too genetically incompatible to produce a fertile egg.
If offspring are produced and they survive they are often sterile and would never be able to breed with any other finch no matter which species they attempt to mate with. Of course before they can mate they must attract a finch of the opposite sex. Attracting a mate isn't always an easy task for them. They don't look or sound entirely like one species or the other. The potential mates simply view them as another finch.
Looking unlike everyone else in the flock has broader complications than simply finding a mate. The coloring of finch species is used to attract a mate and to blend into the environment or confuse a predator. Because these Hybrids don't fit into either species they become extremely easy for a predator to spot and follow. If these finches survive to adulthood they have been very lucky.
On top of it all, the Hybrids are a drain on the species gene pool. While their parents were producing them they aren't passing along their genes to young which would someday produce more. Many species of finch only breed once or twice a year. In many cases they have used their only breeding cycle for the year to produce these rather unique dead end babies. If something should happen to either or both of the parents before they can breed with their own species, that entire family line has been brought to a halt. If the gene pool becomes too small, inbreeding occurs.
The aviary and finch breeding community these hybrids aren't well received. Granted these birds are unique and often odd looking. However they have no value to a breeder. At best you may be able pawn some off on a novice finch keeper or someone who has no interest in breeding finches. A true finch breeder and fancier will probably never pay for a hybrid finch. They simply consume food and produce nothing seeing as most are sterile.
Hybrids are sometimes used to add color to a finch or alter its behavior. This is becoming quite popular in the European Goldfinch community. The Goldfinches are crossed with Canaries to give them a more uniform look, more yellow, and improve their song. These birds are now simply refereed to as Mules. The practice has become so wide spread that many sellers of European Goldfinches at bird fairs must label their birds as Mules or Pure. Mules are worth almost nothing and the Pure are becoming very hard to find and expensive.
This is where the big problem with hybrids comes into play. European Goldfinch's aren't exactly a thriving market in many areas of the world. In places within the US they are even becoming hard to find and the prices are going up. The breeders who are exclusively producing these Mules aren't producing pure European Goldfinches. This is driving their price up and the available gene pool down at an alarming rate. European Goldfinches are still being imported into the US from other countries. This helps bring in new blood, but once the import is stopped we are going to be in trouble.
The export of many species from their native lands is now banned. The number of birds banned species is increasing yearly. There may well come a time when all the genetic diversity we have access to will be what's currently living in the USA. If too many breeders spend their time producing hybrids will we have terrible shortages. This can lead to inbreeding very quickly.
Good Points:
Acceptance of a hybrid into an established aviary isn't usually a problem. Finches either will get along with other species or they won't. It doesn't matter what the other finch looks like in most cases.
Hybrids while being infertile can be useful as foster parents. They are also good if you want to stock an aviary with unique looking and non-breeding finches. Best of all they are often cheap or free. As I stated earlier, they have no value in the finch trade. They are also not recognized by any serious finch organization. Sorry I'm leaking back towards the bad again.
The Society Finch:
Many believe the Society is the result of hybridization. Many others and myself now have their doubts about this. It has been argued that the species was created with the careful hybridization of the Striated Finch and the Indian Silverbill. A slightly more plausible idea is that the Society is simply a domestic version of the Striated Finch (White-rumped Mannikin). They do share some physical and behavioral features with the Society finch where as the Silverbill doesn't.
© lady gouldian finch.com 2011
Why Supplement?
Confused about the role Vitamin Supplements play in your bird's wellness?
Learn More
Why is my Gouldian BALD?
Balding in the Gouldian Finch is common.
Learn More
Hand Feeding Finches
...it's possible but not easy.
Supplies, schedules, formula, brooder, hygiene and weaning.
Learn More
Source : http://www.ladygouldianfinch.com – 5404 Alton Parkway, Suite 357, Irvine CA 92604 800.579.7974
We have been celebrating serving you since 2000!
Copyright 2011 All Rights Reserved
Carp Seed Production In India
The availability of quality seed is prerequisite for rapid expansion and growth of aquaculture. However, uncertainty in timely seed supply is one of the major constraints, says Mr Radheyshyam at the Aquaculture Production and Environment Division, Central Institute of Freshwater Aquaculture Kaushalyagang, India.
Network of Aquaculture Centres in Asia-Pacific
Considering its significance constant efforts have been made to produce large quantity of carp seed every year in increasing trends. For instance, the total fry production in India was estimated at 632 million in 1986-87 which had increased to 18.5 billion in 2002-2003 and in 2005-06 it was over 22.6 billion. Quantified data on larger size fingerlings and/or yearlings are not available, although it is much needed for grow out culture.
Fish seed production includes egg to spawn production for 3 days, spawn to fry nursing for 15-20 days, fry to fingerling rearing for 60-90 days and fingerling to yearling rearing for 8-9 months. Thus the carp seed may be categorised at its final size into spawn (6-8 mm size), fry (20-25 mm size), fingerlings (100-150 mm size) and yearlings (100-200 g weight).
Mass production of carp eggs in a spawning pool
Mass production of carp eggs in a spawning pool.
The distribution system of carp seed is complex and dynamic. Though some of the entrepreneurs produce and supply the fish seed to end users often as a part of complex networks, their supply remains erratic in other part, particularly in rural sectors1. The gap between demand and supply of quality seeds, by and large, remains a daunting task in rural aquaculture development. This can be mitigated, if village farmers produce quality carp seed in their ponds to not only make the access of locally produced and nursed quality seed to the fish farmers but also stimulate and support neighbouring farmers to adopt fish culture within their situation.
Earlier studies indicate that paucity of carp spawn compelled village farmers to stock their ponds with riverine fish seed and due to lack of technical support and basic infrastructure facilities; carp breeding was rarely adopted by farmers. In view of this various attempts have been made to demonstrate carp breeding, spawn to fry rearing and fry to fingerling rearing and fingerling to yearling rearing in rural area. Despite pointed extension focus in this regard, the sustainability aspect of the production of carp seed by the farmers still remains a missing link. Present communication summarises the carp spawn production and seed rearing management by the fish farmers of Orissa by citing examples of some selected cases.
What is rural carp seed production?
Rural carp seed production may be defined as “carp seed production by small-scale households or communities using mainly extensive and semi-intensive management appropriate to existing resource base for their own use and/ or improving their family income” or “carp seed production using technologies adapted to locally available and limited resources of households”. Rural carp seed production is not very capital intensive or input intensive and contributes to rural livelihoods. It is different from more commercially carp seed production systems or entrepreneurial carp seed production.
Evolving rural to entrepreneurial carp seed producers
The system of carp seed production process is a continuum and it is very difficult to strictly divide rural from entrepreneurial fish seed producers. In fact, many farmers who have been involved in subsistence level carp seed production increased their production over the years, with the more inputs and better management skill, resulting in enlarging their resource base and gradually becoming entrepreneurial. For example, a farmer who used to stock spawn in unprepared pond because of not knowing the technique of pond preparation, when came to know, followed the technique strictly and got better recovery and more income. Thus over a period of few years he could afford more inputs and intensifies his management and becomes entrepreneurial seed producer. It is more desirable to make the resource poor farmers entrepreneurial farmers in rural area. Such evolution is already taking place with the time. For instances, farmers of Sarakana village evolved as carp seed entrepreneurs from traditional carp seed producer.
Spawn production in rural area
Common spawn production
In rural areas generally carp spawn are generally produced twice during June-August and January-March of the year, following the adaptive breeding methods.
Pond breeding: Common carp brood fish are reared in composite fish culture ponds. In season, clean aquatic weeds such as Hydrilla / Najaj or water hyacinth are placed in pond’s corners or inside floating bamboo frames in the evening hours. During late night to early morning fish breed naturally and eggs are attached to aquatic weeds. Since water hyacinth is floating, the eggs get attached on the roots only. The egg loaded aquatic weeds are collected in morning hours and kept for incubation in hatching hapas or directly spread in well prepared nursery ponds. However, in nursery spread eggs the spawn survival is very poor than hapa hatching. This method has certain disadvantages like: difficult to estimate eggs, egg predation by pond animals, poor egg fertilisation etc.
Release of carp spawns in incubation pool.
Release of carp spawns in incubation pool.
Hapa breeding: Brood fish are reared either in separate ponds or in composite fish culture ponds. Brood fish are netted out to segregate mature males and females. They are weighed and kept in breeding hapa containing suitable egg collectors in evening hours. Generally 3-4 kg Hydrilla/ kg female fish is used as egg collector. Males and females are kept in ratio of 1:1 by weight. They breed naturally in hapa after 6-8 hrs. In less suitable condition fishes are injected with inducing hormones to ensure breeding. After spawning, the females are weighed to estimate the egg release. About 12-15 per cent of the weight difference goes towards faecal matter of fish and rest weight difference is due to egg release in ovary. One gram weight difference in ovary provides an estimate of 700 egg release. Egg attached 2-4 kg Hydrilla is spread per inner hatching hapa. Depending on water temperature, hatching takes place in 2 days and inner hatching hapas are removed in 3 days. After 4-5 days, spawn are collected for stocking in nursery ponds.
Hatchery breeding: Some of the village hatchery owners use breeding pools for common carp spawning. They use nylon threads or plastic threads or plastic nets or Hydrilla or water hyacinth as egg collectors. Egg incubation is carried out in hatching pools.
Indian and exotic major carp spawn production
Hapa breeding: In remote villages brood fish are grown in composite fish culture ponds. During monsoon season they are netted out and fully mature males and females are selected. Breeding hapas are fixed in composite fish culture ponds having common carps. Presence of common carp, prawns and crabs cause severe damage to carp eggs in breeding hapas. Hence, to avoid hazards of loss of viable eggs, the breeding hapas are fixed inside the net enclosure. Generally for one female two males are used. Intra-muscular and/or intra-peritorial injection is administered to brood fish during June-October.
Females are injected with PG extract or glycerine extract of PG twice but males are injected only once. First dose is given in the evening hours to female @ 5-6mg/kg and second dose after 4-6 hours of fi rst injection @ 8-16 mg/kg. Males are injected at the time of second dose of female @ 4-5mg/kg male. Presently synthetic hormones (ovaprim or ovatide) are used as inducing agents in rural areas. Both the males and females are injected only once.
These synthetic hormones are administered @ 0.2-0.5 ml/ kg female and 0.1-0.2 ml/kg male. After 4-6 hours of injection fish spawn. Fertilised eggs are identified and quantified at comma stage of embryos and hatching are done using hapa hatching device. Spawn are collected after 72-80 hours of hatching by filtering with inner hatching hapa with the spawn recovery of only 24-44 per cent of the fertilised eggs. The low recovery of spawn from hapa hatching device could be due to a combination of factors such as cutting of hapas by crabs and/or large freshwater prawns, entry of unwanted fishes in hatching hapas8, presence of predatory cyclopoid copepods in hatching hapas and sudden change in water temperature, depletion of DO content, water bloom and cyclonic weather.
Hatchery breeding: For hatchery breeding, brood stocks are maintained in separate ponds by stoking 1-3t/ha brood fish under scientific management. Brood fishes are injected with inducing hormones as mentioned in hapa breeding. In rural areas the spawning is done in breeding hapa and/or spawning pool but hatching is done in incubation pools.
Two-three year old carps weighing 2-5 kg are the best for hypophysation. “Eco-hatchery” is used by the village entrepreneurs. It includes overhead tank, spawning pools, egg collection chamber, incubation pools and spawn collection chamber. An overhead tank is generally made on the roof of single or double storied building and a water holding capacity of 5000 litre can supply water to spawning and incubation pools. Depending upon the requirements, the sizes of spawning pools vary. Spawning pool is 8-9 m diameter and 1.0-1.5 m deep with the provision of water circulatory system and shower.
Farmers use 20-30 kg female per spawning pool and produce 250-400 litres of carp eggs in one operation. These eggs are incubated in 3-5 hatching pools. Incubation pools are 3-4m inside diameter and 1 m deep. Generally 1 egg is incubated in one ml water. During egg incubation, farmers maintain water flow @ 2.5 l/sec. initially, @ 2.0 l / sec at twisting movements of embryos and @ 3.5 l/sec after hatching to get better spawn recovery. Farmers harvest 800,000 to 1,000,000 spawn/pool/operation. KVK/TTC, CIFA designed and fabricated portable FRP carp hatchery in 1989 with the maximum spawn recovery of 3,000,000 lakh / operation/pool, now modified and commercialised by CIFA and it is used by the village entrepreneurs to produce carp spawn. From hatchery breeding farmers get 80-95 per cent recovery from the viable eggs. By adopting circular carp hatchery some of the rural fish farmers changed into entrepreneurial seed producers.
Success cases of carp spawn production
Carp spawn production at Sarakana: Farmers from the Sarakana village started carp spawn production in 1987 with common carp and produced 3.5 lakh spawn in hapa - breeding. Gradually they learnt the induced breeding techniques of Indian major carps and exotic carps in hapa. Carp spawn production increased to 1,440,000-8,555,000 up to 1995.
The spawn recovery was poor and ranged between 24-44 per cent. To mitigate the problems of poor recovery of spawn in hapa, they have been motivated by KVK/TTC, CIFA to construct a cemented circular hatchery in 1995 which resulted higher spawn recovery of 74-85 per cent from 1996 onwards. This resulted in producing 15,750,000-31,950,000 spawn of Catla catla, Labeo rohita, Cirrhinus mrigala, Cyprinus carpio, Ctenopharyngodon idella and Hypophthalmychthys molitrix annually. To meet the market demand of carp spawn in the region, they ploughed back their hard earned money to construct another carp hatchery with higher production effi ciency. As a result of which they are able to produce 100-150 million carp spawn annually. This suggests that traditional seed production in rural area transformed into entrepreneurial seed production by utilising the improved technology. They produce carp seed not only to meet the market demands but also earn handsome income and employment.
Carp fry production in rural area
Carp fry production in rural area.
Carp spawn production at Kantapada: Farmers from Kantapada village initiated carp spawn production in 1996 using hapa breeding device. With spawn recovery of 25-40 per cent of viable eggs, they produced 40, 50, 67and 42 lakh spawn during 1996, 1997, 1998 and 1999. After realising the poor spawn recovery, farmers constructed one circular carp hatchery and now they are producing 40-60 million carp spawn annually.
Carp spawn production at Bhatapadgarh: Carp breeding was started with hapa breeding with the technical guidance of CIFA, Kausalyagang in stored rain water in ponds constructed at hilly terrains during 2002. Farmers have been trained through participatory approach in carp breeding skills. During skill learning farmers could produce 1,100,000 carp spawn with 20-40 per cent recovery in hapa breeding. Meanwhile, they constructed one commercial carp hatchery during 2003 and made it operational through the technical guidance of the author in 2004. Now farmers are producing 50-110 million spawn of C. catla, L. rohita, C. mrigala, C. carpio, L. calbasu, C. idella and H. molitrix every year at the hilly terrains to meet the regional demand.
By seeing the economic profitability in carp spawn production, many of the neighbouring farmers and entrepreneurs have constructed carp hatchery to produce carp spawn to meet the local carp seed demand.
Carp fry and fingerling rearing in rural areas
In rural area, spawn to fry nursing is carried out in smaller ponds of 0.02-0.05 ha (0.5-1.0m depth). In same perennial ponds fry, fingerling and/or yearlings are reared in succession during June-July, August-November and December-June respectively. Alternatively the ponds are stocked with carp fry and rearing of fingerlings and yearlings are continued in succession.
For rearing larger size carp fingerlings 0.05-0.1 ha with an average depth of 1.0-2.0m are preferred. Ponds shaded by trees are rendered unproductive by reduced sunlight. Accumulation of leaf litter and an excessive organic load in the pond further deteriorates water quality, adversely affecting carp and carp food organisms. At times, masses of foamy brown/white frog eggs, which tend to fall into ponds during rains, caused a proliferation of tadpoles.
Therefore, marginal trees and bushes are cleared before launching the seed raising programme. Pond embankments are renovated with the provision of secured inlet and outlet. Since backyard ponds are shallow and small, aquatic weed clearance is completed manually by rural farmers. Predatory animals/ fishes and weed fishes are eradicated by de-watering and drying the ponds or application of suitable piscicides. Raw cattle dung is applied as basal manure in ponds. To enhance the fertilisation effect liming is done. For sustained production of natural fish food organisms a mixture of de-oiled cake, cattle dung/ bio-gas slurry and single super phosphate or a multiplex pre mineral mixture and vitamins are used in liquid forms before 4-5 days of spawn stocking.
Fry are harvested and/or thinned in phases according to the local demand, allowing an extended period of rearing (14-44 days) in rural area. Prolonged retention of fry in nursery ponds adversely affects the fry survival. Fry recovery is 20-40 per cent. Stocking spawn at shallow water depth (35-45 cm) followed by phased increase of water level at 3 - 4 days intervals, results higher fry recovery of 50-70 per cent. Fortification of micro-nutrients in artificial feeds is also enhances the growth and survival of fry. A commercially available multiplex pre-minerals mixture with vitamins accelerates plankton production and fry survival in nursery ponds. In case ponds are used for fry rearing, fry are harvested by repeated netting on day 15-20 of stocking.
At times, two crops of fry are taken. After fry harvesting, the ponds are fertilised with the mixture of above manure to produce adequate natural fish-food organisms. On day 2 or 3 of fertilisation, the fresh fry are stocked along with residual fry in such a way to maintain the density of 300,000-500,000/ ha. Later a mixture of above fertilisers is applied in liquid form at weekly or fortnightly intervals. Fingerlings are also fed traditionally and harvested by repeated netting after three months of rearing.
Success cases of fry and fingerling production
Fry and fingerling production at Sarakana village: Farmers from the Sarakana village started carp fry raising in one pond of 0.08ha and produced only 220,000 fry and 40,000 fingerlings. High profitability in fry and fingerling rearing work encouraged the farmers to invest money for creating more facilities by constructing two other ponds in 1988 and produced 384,000 fry and over 100,000 fingerlings. Since then every year the farmers expanded their activities by excavating new ponds and at preset 23 ponds of 0.02-0.1ha each are available for fry and fingerling production. Now they are producing 4,300,000-6,000,000 lakh fry and 440,000- 570,000 fingerlings every year.
Fry and fingerling production at Kantapada village: In this village fish seed nursing was initiated in 1983 by using 12 nursery ponds. Ponds were prepared and stocked @ 30-50 lakh spawn/ha. The fry were harvested after 30-45 days with the recovery of 15-30 per cent. With the time farmers acquired scientific management practices and expanded rearing area to 20 ponds (2.0 ha) gradually. Farmers are harvesting carp fry within 12-20 days with the recovery of 35-60 per cent. Multicropping of fry production is also done. They are able to harvest 3,000,000-7,600,000 fry annually. The same ponds are used for fingerling rearing with the production of over 300,000-600,000 fingerlings every year.
Fry and fingerling production at Bhatapadagarh village: Terrace type a series of 17 nursery and rearing ponds (0.05-0.17ha) are constructed with a network of inlets and outlets systems during 2003 to store huge quantity of water fl owing in from the hilly terrains.These ponds were prepared by manuring, liming and insect control and stocked with carp spawn @ 3,000,000-6,000,000/ha. Ponds were harvested after 20-30 days of rearing with the recovery of 20-60 per cent yielding about 5,900,000 fry from July to September in 1 or 2 crops. After developing confidence in economic profitability, the farmers also started using even large size ponds of 0.5-0.7ha for stocking carp spawn at shallower depth followed by phased increase of water level for commercial fry and fingerling production. They are producing 6,000,000- 15,000,000 fry and 100,000-800,000 fingerlings of catla, rohu, mrigal, calbasu, common carp, silver carp, and grass carp every year for supply in the region.
Large sized fingerling and yearling production
Yearlings are produced traditionally in village ponds. When farmers fail to sale their fingerlings and they continue to rear them up to May-June. Before monsoon, when ponds are prepared for next fry rearing crops, farmers harvest stunted fish for consumption as they are grown with reduced nutrient uptake. But now a days with the increased awareness of yearlings significance as stocking materials, it is being sold at pond site for grow out fish culture.
When stunted fingerlings are kept on a high quality diet they grow rapidly leading efficient body weight. Some of the village fish farmers produce yearlings and/or stunted fingerlings with improved management on commercial scale. In this, the fingerlings stocked in well prepared ponds at high density July-August. Yearlings are also reared by stocking appropriate carp fingerlings along with residual stock of fingerlings. During culture period ponds are fertilised monthly once. Fingerlings are fed with the mixture of ground nut oil cake and rice bran in the ratio of 1:1 by weight @ 4-6 per cent of the body weight. Complete harvesting of yearlings is done by repeated netting from May-June. Adopting this management the farmers of Kantapada and Bhatpadagarh are producing 3-5 tonnes of yearlings every year.
Acknowledgements
Author wishes to express his gratitude to Dr. A. E. Eknath, Director of Central Institute of Freshwater Aquaculture, Kaushalyagang and Dr J.K. Jena, Aquaculture Production and Environment Division for their constant encouragement and inspiration for this work. Thanks are also due to Dr. H. K. De, Sr. Scientist for critically going through the manuscript and improving suggestions.
Source : http://www.thefishsite.com/articles/936/carp-seed-production-in-india
5M Enterprises Ltd., Benchmark House, 8 Smithy Wood Drive, Sheffield, S35 1QN, England.
5M Enterprises Inc., Suite 4120, CBoT, 141 West Jackson Boulevard, Chicago, IL, 60604-2900, USA.
Contact TheFishSite | Terms and Conditions | Privacy Policy | Disclaimer
Co. Registration 3332321 - VAT No. 100 1348 86 - A Benchmark Holdings Ltd. Company
Jumat, 28 Oktober 2011
Blood Sugar Regulation
Ads by Google
Daido Industries INC.
Blood Bank Refrigerator / Freezer
Incubator / Agitator / Cold Bench
www.daido-ind.co.jp
Osmometer made in Germany
Fast and effective measuring -
competent after sales service!
www.gonotec.com
Most cells in the human body use the sugar called glucose as their major source of energy. Glucose molecules are broken down within cells in order to produce adenosine triphosphate (ATP) molecules, energy-rich molecules that power numerous cellular processes. Glucose molecules are delivered to cells by the circulating blood and therefore, to ensure a constant supply of glucose to cells, it is essential that blood glucose levels be maintained at relatively constant levels. Level constancy is accomplished primarily through negative feedback systems, which ensure that blood glucose concentration is maintained within the normal range of 70 to 110 milligrams (0.0024 to 0.0038 ounces) of glucose per deciliter (approximately one-fifth of a pint) of blood.
Negative feedback systems are processes that sense changes in the body and activate mechanisms that reverse the changes in order to restore conditions to their normal levels. Negative feedback systems are critically important in homeostasis, the maintenance of relatively constant internal conditions. Disruptions in homeostasis lead to potentially life-threatening situations. The maintenance of relatively constant blood glucose levels is essential for the health of cells and thus the health of the entire body.
Major factors that can increase blood glucose levels include glucose absorption by the small intestine (after ingesting a meal) and the production of new glucose molecules by liver cells. Major factors that can decrease blood
The homeostatic regulation of glucose concentrations.
The homeostatic regulation of glucose concentrations.
glucose levels include the transport of glucose into cells (for use as a source of energy or to be stored for future use) and the loss of glucose in urine (an abnormal event that occurs in diabetes mellitus).
Insulin and Glucagon
In a healthy person, blood glucose levels are restored to normal levels primarily through the actions of two pancreatic hormones , namely insulin and glucagon. If blood glucose levels rise (for example, during the fed or absorptive state, when a meal is digested and the nutrient molecules are being absorbed and used), the beta cells of the pancreas respond by secreting insulin. Insulin has several notable effects: (1) it stimulates most body cells to increase their rate of glucose uptake (transport) from the blood; (2) it increases the cellular rate of glucose utilization as an energy source; (3) it accelerates the formation of glycogen from glucose in liver and skeletal muscle cells; and (4) it stimulates fat synthesis (from glucose) in liver cells and adipose (fat) tissue. These effects collectively cause a decrease in blood glucose levels back to normal levels.
If blood glucose levels fall below normal levels (for instance, during the post-absorptive or fasting state, when nutrients from a recently digested meal are no longer circulating in the blood, or during starvation), insulin secretion is inhibited and, at the same time, the alpha cells of the pancreas respond by secreting glucagon, a hormone that has several important effects: (1) it accelerates the breakdown of glycogen to glucose in liver and skeletal muscle cells; (2) it increases the breakdown of fats to fatty acids and glycerol in adipose tissue and, consequently, the release of these substances into the blood (which cells can thus use for energy); and (3) it stimulates liver cells to increase glucose synthesis (from glycerol absorbed from the blood) and glucose release into the blood. These effects collectively cause an increase in blood glucose levels back to normal levels.
In addition to insulin and glucagon, there are several other hormones that can influence blood glucose levels. The most important ones are epinephrine, cortisol, and growth hormone, all of which can increase blood glucose levels.
Diseases and Blood Sugar Regulation
Glucose levels above or below the normal range are indicative of the presence of disease states. For example, elevated glucose levels are present in diabetes mellitus, Cushing's syndrome, liver disease, and hyperthyroidism, while decreased glucose levels are present in Addison's disease, hyperinsulinism, and hypothyroidism.
The most prevalent of these diseases is diabetes mellitus. There are two types of this disease: Type I (insulin-dependent or juvenile-onset) diabetes mellitus, and Type II (noninsulin-dependent or maturity-onset) diabetes mellitus. In Type I diabetes, pancreatic beta cells are destroyed by an erroneous attack by the body's own immune system, and thus insulin secretion is reduced to negligible levels. In Type II diabetes, insulin secretion is not reduced; however, there is a reduced sensitivity of target cells to insulin, a phenomenon known as insulin resistance.
Source: http://www.biologyreference.com
Copyright © 2011 Advameg, Inc.
Rabu, 26 Oktober 2011
science.howstuffworks.com
Photosynthesis
Photosynthesis, the conversion of light energy into chemical energy in cells that contain chlorophyll, a green pigment. Photosynthesis occurs in most plants and algae and in some bacteria and protozoans. The process is also called carbon fixation, because it includes the production of carbon compounds that store the chemical energy for use in cell growth. These compounds—mainly sugars and starches, collectively called carbohydrates—also serve as building blocks for more complex foods, such as fats and proteins. For photosynthesis to occur, water, carbon dioxide, chlorophyll, and light are necessary.
The Reactions of Photosynthesis
Two main chemical reactions occur in photosynthesis. One takes place only in the presence of light and is called the light reaction; the other can occur with or without light and is called the dark reaction.
The Light Reaction has the following steps:
* Light enters a cell and is absorbed by chlorophyll. The light's energy raises the energy level of some chlorophyll electrons, freeing them from the chlorophyll molecules.
* Molecules of water (H2O) from the environment take part in chemical reactions in the cell. Electrons from the hydrogen atoms in each of these water molecules are attracted to the chlorophyll molecules lacking the electrons freed in step 1. This attraction causes the water molecules to break apart into oxygen atoms, protons, and electrons. The oxygen atoms join together in pairs, forming molecules of oxygen. Oxygen molecules, called free oxygen, are released into the environment.
* The electrons freed from the chlorophyll molecules and the protons freed from the water molecules take part in chemical reactions in the cell. These reactions result in the production of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide diphosphate (NADPH2).
The Dark Reaction
The chemical energy possessed by ATP and NADPH2 is used in making carbohydrates from hydrogen and carbon dioxide. (The carbon dioxide is obtained from the environment.) The carbohydrates then possess the chemical energy given up by ATP and NADPH2.
The generalized, overall chemical equation for photosynthesis is:
6CO2+12H2O + light → C6H12O6 + 6O2 + 6H2O
The carbohydrate in this equation (C6H12O6) is glucose, a simple sugar. Glucose is only one of several compounds that can be formed by photosynthesis.
Importance of Photosynthesis
Photosynthesis is the most important chemical process for life. Through photosynthesis, the sun's energy is made available to all organisms. For example, when an animal eats a plant, it obtains chemical energy that the plant acquired through photosynthesis; when a second animal eats a plant-eating animal, it obtains some of the chemical energy that the first animal obtained by eating plants.
As organisms respire, they take in free oxygen and give off carbon dioxide. Respiration is dependent on photosynthesis because photosynthesis is the source of virtually all the free oxygen in the atmosphere and in bodies of water. In addition, photosynthesis removes carbon dioxide from the atmosphere and from bodies of water. If this carbon dioxide were not removed, it would eventually smother the organisms that produce it.
Coal and petroleum, composed of the remains of various kinds of organisms, contain energy that was captured from the sun's rays by photosynthesis millions of years ago.
© 1998-2011 HowStuffWorks, Inc
Senin, 24 Oktober 2011
http://www.tripmondo.com
Explore Jamur in Bangladesh
Data peta ©2011 Google - Syarat Penggunaan
Peta
Peta
Satelit
Hibrida
Medan
Jamur in Dhaka Division is located in Bangladesh - about 10 mi (or 17 km) North-West of Dhaka, the Bangladeshi seat of the government.
Local time in Jamur is Tuesday 12:35:16 PM. The local timezone is "Asia / Dhaka" with an UTC offset of 6 hours.
Dhaka Tongi Purba Naodoba North Baiksa and Syamgaon are destinations relativly nearby and may be interesting to visit on your trip.
According to our current information, there is one airport in the nearby. Have a look at our photo collection to get a view of what this places is like. Want to see what it's like?. No Problem with these videos related to Jamur. Need hints for finding things to see and want to get to know more about this place? Check our related Jamur attractions and wikipedia articles page. To get a first impression of the local parties, events and conferences, we looked up some events from eventful.
National flag of Bangladesh
Jamur Gallery
Itabhara Bridge, Hemayetpur-Keraniganj
Itabhara Bridge,...
Uploaded by TheFirstKind on panoramio.com
Uploaded by kadir441272 on panoramio.com
* About Tripmondo & FAQ -
* Privacy Policy -
* © 2008 - 2011 Tripmondo
* Tripmondo is a private and experimental project - A hommage to free, open and global online data sources.
00
Senin, 17 Oktober 2011
evolution.berkeley.edu
Natural selection
Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift.
Darwin's grand idea of evolution by natural selection is relatively simple but often misunderstood. To find out how it works, imagine a population of beetles:
1. There is variation in traits.
For example, some beetles are green and some are brown.
Color variation in these beetles
2. There is differential reproduction.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
Differential reproduction
3. There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
Heredity of the traits of the beetles who survive
4. End result:
The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
Eventually, the advantageous trait dominates
Download this series of graphics from the Image library.
If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. It is as simple as that.
previous
Genetic drift
next
Natural selection at work
Mechanisms
page 12 of 22
<< previous | next >>
Take a sidetrip
See how the simple mechanisms of natural selection can produce complex structures, learn about misconceptions regarding natural selection, or review the history of the idea of natural selection.
Evo examples
Learn more about natural selection in context:
* Angling for evolutionary answers: The work of David O. Conover, a research profile.
* Battling bacterial evolution: The work of Carl Bergstrom, a research profile.
Teach this
Teach your students about natural selection:
* Clipbirds, a classroom activity for grades 6-12.
* Breeding bunnies, a classroom activity for grades 9-12.
Find additional lessons, activities, videos, and articles that focus on natural selection.
Home | About | Copyright | Credits | Contact | Subscribe | Translations
Read how others have recognized the Understanding Evolution website.
http://www.buzzle.com
Importance of a Library
Why is a library such an important place for many people? Read on to find out the importance of a library...
The more that you read, the more things you will know. The more that you learn, the more places you'll go. ~ Dr. Seuss
Elixir for a bad mood, lifesaver while completing school assignments and a loyal companion by the bedside - books are a wonderful influence in your life. Be it your copy of the Cat in the Hat, your reference books in the university library or novels that you curl up in the night - books enrich our lives and help us become the people that we are. People who are fond of reading will agree with the fact that a library is perhaps the most peaceful place on earth! If you are contemplating the thought of having a library in your home, here are some reasons, which will convince you that your decision is completely right! Here are some points about the importance of libraries:
Inculcating the Habit of Reading
Reading is regarded as one of the most enriching habits for the simple reason that it is not just a hobby or a pass time that entertains you, but it is also an educational activity and hence brings to you a vast reservoir of knowledge. Reading increases the drive for knowledge and inspires people to gain more information. Thus a library is a treasure of valuable books for the people to use and gain from it.
Learning Experience for the Children
A library is a very important aspect in the learning process of your child. The extensive genre of children's literature is an essential part of the growing up process. In case you have enough resources, it is always advisable to have an in-house library. If you think you cannot afford a library at home, you can always visit a public library. Most of the public libraries are keeping with the times and equipped with facilities like CDs and even computers.
Reference for School/Colleges
The quintessential library is a boon for the students in schools and colleges. There exist a large number of reference books that provide information about wide ranging subjects are a must for students to understand the concepts in their curriculum. The reference books often provide in depth information about various subjects and thus help in the process of education.
Advice on Important Subjects
There are large number of books that provide advice about various topics like business, health, travel, food and careers. These books serve as a great source of advice. Many people make it a point to read and go through these books before taking important decisions in their life. Thus libraries are also helpful for people who are looking for information about specific subjects. For example a person who is planning to travel to a particular place would like to read about that destination.
Wholesome Information
A library usually has a good collection of encyclopedias, dictionaries and maps, which are a source of extensive information and references for people. The encyclopedias are a vast source of information about all the topics under the sky. There also exist specialized dictionaries like medical dictionaries, literature dictionaries or business dictionaries, which provide information about specific terms used in specialized fields.
Entertainment and Fun
In addition to the above mentioned points, libraries are also a host to large number of books that are a source of entertainment for us. Fiction books, which include various genres like comedy, thriller, suspense, horror or drama, are tremendously popular within readers of varying age groups.
Libraries are thus a source of entertainment and education for youngsters as well as adults. A library not only helps to inculcate the habit of reading but inculcates a thirst for knowledge, which is makes a person humble and open to new ideas throughout his/her life.
By Uttara Manohar
Ads by Google
HSBC - Global Education Free overseas education consultation from HSBC Premier www.hsbc.co.id/global-education
Free Digital Library Search the global online library on ethics. Sign up for free. www.globethics.net/library
What is RTI? Free information for parents on Response to Intervention! NCLD.org/parent-guide-to-RTI
Cat Generator Sets Diesel Or Gas Powered 7 To 16000kW Standby And Prime Applications www.catelectricpowerinfo.com
Join Digitalkoot now Help the Finnish National Library digitize archives by playing games! www.digitalkoot.fi
RFID System for Library LibBest Library RFID Management, Security System www.rfid-library.com
©2000-2010, 2011 Buzzle.com® · All rights reserved.
Jumat, 14 Oktober 2011
http://www.globalspec.com
Biotechnology Products
Biotechnology products used in research and development and include products such as tissue cultures, prepared media, biosensor arrays and reagents.
What are you looking for in Biotechnology Products?
Capillary Electrophoresis Equipment (42 companies)
Capillary electrophoresis (CE) is a family of related separation techniques that use narrow-bore fused-silica capillaries to separate a complex array of large and small molecules. Learn more about Capillary Electrophoresis Equipment
Flow Cytometers (21 companies)
Flow cytometers use laser beams to characterize single cells as they pass by at high speed.
Search by Specification | Learn more about Flow Cytometers
Microplate Coating Systems and Microplate Processing Systems (39 companies)
Microplate coating systems and microplate processing systems are used to prepare samples for drug discovery or chemical analysis applications. Learn more about Microplate Coating Systems and Microplate Processing Systems
Microplate Readers (77 companies)
Microplate readers are designed to scan, analyze and obtain numerical results from chemical reactions conducted within microplates.
Search by Specification | Learn more about Microplate Readers
Microplate Washers (42 companies)
Microplate washers combine buffer dispensing and aspiration cycles to remove reagents from sample wells.
Search by Specification | Learn more about Microplate Washers
Biological Indicators (39 companies)
Biological indicators are used to monitor and evaluate the effectiveness of sterilization. Learn more about Biological Indicators
Biosensors and Microarrays (35 companies)
Biosensors, microarrays, biochips and lab-on-chip (LOC) products are microscale devices for biological, biochemical and chemical arrays. They consist of microfluidic channels and a biodetector or microsensor arrays. Learn more about Biosensors and Microarrays
Biometric Sensor Chips (23 companies)
Biometric sensors are semiconductors with embedded algorithms that are used in security systems or environments for user authentication. Learn more about Biometric Sensor Chips
Densitometers (35 companies)
Densitometers measure the optical, photographic or area density of a material. They are used in a wide range of applications, from film processing to medical scans. Learn more about Densitometers
DNA Synthesizers (27 companies)
DNA synthesizers are used to custom build DNA molecules to contain a sequence of interest. Learn more about DNA Synthesizers
Gel Electrophoresis Equipment (66 companies)
Gel electrophoresis equipment, instruments and supplies are used to separate macromolecules, either nucleic acids or proteins, on the basis of size, electric charge, and other physical properties. Learn more about Gel Electrophoresis Equipment
Organic Chemicals (1403 companies)
Organic chemicals are chemical compounds that contain carbon in their molecular structure.
Search by Specification | Learn more about Organic Chemicals
Pharmaceutical and Medical Gases (64 companies)
Pharmaceutical gases and medical gases are specialized for medical, drug processing or biotechnology applications. Learn more about Pharmaceutical and Medical Gases
Thermal Cyclers (50 companies)
Thermal cyclers are laboratory instruments capable of generating and maintaining specific temperatures for a defined period of time. Learn more about Thermal Cyclers
Of Interest
* Related to Biotechnology Products
* Search By Part Number
* Application Notes
* Find Product Announcements for Biotechnology Products
Product Announcements
Mono-, Di- and Tri-Propargylamines
Mono-, Di- and Tri-Propargylamines
Arkalon Chemical Technologies, LLC
Gelatin for Photographic Processing
Gelatin for Photographic Processing
Eastman Kodak Company
High-Purity Low Endotoxin Carbohydrates
High-Purity Low Endotoxin Carbohydrates
Ferro Corporation
See more product announcements for Biotechnology Products
e-newsletter sign up
product alert sign up
* Home
* About GlobalSpec
* Advertise With Us
* Site Map
* Top Categories
* Terms of Use
* Privacy Policy
* Link To Our Site
* Submit a Site
* Recommend This Site
©1999-2011 GlobalSpec. All rights reserved. GlobalSpec, the GlobalSpec logo, SpecSearch, The Engineering Search Engine and The Engineering
Web are registered trademarks of GlobalSpec, Inc. The Engineering Toolbar and DesignInfo are service marks of GlobalSpec, Inc.
No portion of this site may be copied, retransmitted, reposted, duplicated or otherwise used
without the express written permission of GlobalSpec Inc. 30 Tech Valley Dr Suite 102, East Greenbush, NY 12061
http://www.canine-genetics.com/Mutation
The Canine Diversity Project
ELIMINATING MUTATION
THE IMPOSSIBLE DREAM
Though it is not practical to eliminate all deleterious mutation, the incidence of affected individuals may be significantly reduced through a combination of intelligent breeding practice and the development of DNA tests.
Why do we have mutations?
Mutations are changes in an organism's DNA that potentially affect the correct functioning of genes. They occur naturally due to replication errors, mispairing of homologous chromosomes, or through unavoidable exposure to natural radiation (e.g., cosmic rays). Mutations can occur anywhere in the DNA and in any cell. They are heritable only when they occur in the germ cells (eggs and sperm), but mutations in the DNA of other (somatic) cells may lead to cancer. Even though the DNA replication enzymes are very accurate, and there are also supplementary systems for detecting and correcting damage, no system is perfect. We should, therefore, recognize that some level of mutation is inevitable. However, the mutation rate is increased by radiation, including ultraviolet light, and exposure to certain toxic chemicals. We can, therefore, take some precautions to minimize the risk..
The mutation rate for dogs cannot be determined readily, but from indirect evidence and extrapolation from other species, geneticists believe that mutation rates are normally on the order of 1 in 100,000 or less. For a sexually reproducing mammal, that would mean a new mutation in a particular gene would likely not occur more often than once in every 100,000 gametes. That may not seem like a high probability, but consider that most mammals are estimated to carry 80-100,000 genes. This suggests that every individual born has a good chance of carrying one new mutation in some gene.
What happens to new mutations?
Identical mutations are unlikely to occur simultaneously in the same gene from both parents (probability: < 1 in 10 billion), so any progeny will be heterozygous. (The exception being sex-linked genes, as the X and Y chromosomes are not homologous.) Dominant mutations will be expressed and any that are deleterious will be eliminated almost immediately from the population. If the mutation is advantageous, and this advantage is noticed by breeder or "nature", the mutation may survive and its frequency gradually increase. If a mutation neutral, which is to say, neither good nor bad (just different), its survival will be determined by "genetic drift". New recessive mutations remain hidden from selection until they reach a frequency where some homozygous individuals begin to appear. However, this does not prevent drift loss, which doesn't depend on phenotype.
Drift is a consequence of the random nature of genetic events. For example, if you breed a brown bitch to a black dog carrying brown, you would expect ½ the progeny to be black and ½ brown, but probably wouldn't be too surprised if you got 7 blacks and 3 browns in a litter of 10. It works the same way for any gene that has two or more alleles. Suppose that we have only one black dog (Bb), all the rest being bb. The one Bb dog may pass the B allele to none or all of his progeny, or to any number in between. If he has more than 5 black progeny, the frequency of black will go up providing all contribute equally to the next generation. In subsequent generations the frequency may drift even higher, or back down.
In a large population, the frequency will tend to fluctuate by only a small amount. However, small populations are inherently unstable and, if other factors don't intervene, one allele will eventually take over. This is called fixation. How long this takes depends on population size. With a rare breed, fixation may easily occur within 25 generations (~ 100 yrs.)
Many recessive mutations persist for a few generations at low levels before being lost again. Only very rarely do they reach a significant level in the population (> 1 in 1000). In terms of estimates of genetic diversity based on average heterozygosity, these genes are effectively monomorphic, as a screen of 50 or 100 individuals from the population would generally fail to reveal any differences for the majority of the these loci. When two individuals appear to carry the same mutation, it may well be due to independent mutations. However, unless there is some common ancestry, the chance of producing affected progeny should be no more than 1 in a million. [Notably, in the first study of an "inborn error of metabolism", Garrod (1902) observed that "among the families of parents who do not themselves exhibit the anomaly a proportion corresponding to 60 per cent are the offspring of marriages of first cousins." He estimates that only about 3% of all marriages are between first cousins.]
These estimates assume equal use of all individuals in the population, and we all know how common that is. If a particularly popular sire produces 10 times his "share" of sons and daughters, whatever deleterious allele(s) he carried will get a substantial boost in the next generation. A new mutation may be promoted from one-of-a-kind to moderately frequent in this way. As long as we insist on making mate choice a popularity contest, we risk introducing new problems as fast as we can develop tests for the old ones.
Genetic "load" and the founder effect
The human population carries at least 2500 deleterious mutant genes (or, more correctly, alleles of genes) causing significant health problems. For the most part they are fairly evenly distributed in the population. For the entire Canis familiaris population, the situation is likely fairly similar. Each individual is estimated to carry a "genetic load" of three or four "lethal equivalents", which implies recessive alleles that would kill of the bearer if they were homozygous. As long as they are recessive, they should not cause problems.
However, consider what happens if we form a subpopulation by choosing 10 individuals from a much larger population. Though these individuals will not carry the vast majority of the unwanted deleterious recessive alleles found in the wider population, the few they do carry will be promoted instantly from rare alleles (0.1% or less) to at least 5% in our example (or more generally, 1/2N, where N is the number of founders).
Because random drift has a greater impact on a small population, the population needs to grow rapidly, to at least several hundred breeding individuals, so as to minimize the loss of valuable alleles. During this time, we should select cautiously. While it is true that fixing "type" is one of the prime objectives of purebred dog breeders, too rigorous selection during the early generations increases the possibility of accidental loss of a valuable gene closely linked to one of the genes under selection. Dalmatians, for example, are all deficient in an enzyme required for correct uric acid metabolism. The mutant gene appears to be closely linked to one of the genes for the characteristic spotted pattern and was likely inadvertently fixed when early breeders selected for that pattern (Nash, 1990).
Recognizing mutation
Though, at an allele frequency of 5%, affected individuals should only make up about 0.25% of the population, this would be a good time to stop it from increasing further. However, would a mutation occurring at that frequency be recognized as such? If we are talking about breed with average litter size of four, then we are only looking at about one litter in 100 with one affected puppy. If there have been no other reports, the breeder may simply write it off as "one of those things". In a breed with larger litters, the probability of two or more affected pups occurring in the same litter is greater, but even in these cases, lack of exchange of information between breeders and lack of education in genetics may result in a failure to identify the problem as genetic.
Selection
Selection is only effective if we are dealing with easily recognized phenotypes. However, undesirable mutations are not always that accommodating. There is a full range of possibilities from silent mutations, that have no noticeable effect on proteins coded for, to mutations that fail to make any functional product. There is even a small possibility of improvement. Those, and the silent class, are no threat to us. However, those that prevent normal function but do not eliminate it completely are likely to present a substantial problem. One example is the vWD mutation in Dobermans. This mutation eliminates 85-90% of the active clotting factor, but this low level is still sufficient to protect a homozygous affected individual from excessive bleeding in most situations. A dog that is "lucky" enough to avoid a major injury or surgery may not be recognized and may even be bred. Consequently, the frequency of the mutant allele rose to slightly over 50% in the population (Brewer, 1999).
This should not be regarded as an exception. Fewer than one in three mutations appear to be fully lethal, and that the others cover the full spectrum from the 0-100% activity. In addition to dealing with a handful of easily-recognized genetic diseases in a breed, we are also likely to be dealing with scores of others that reduce fitness but present no obvious phenotype that can be used to identify them. If we can miss a gene that is only 10-15% functional, how well are we likely to do with those that retain 80 or 90% of their normal function?
Why should this be a problem?
In a small population, drift inevitably leads to fixation for one allele. Computer simulations show that if we start with a neutral allele with a frequency of 5% in the population, as would be the case if it was originally carried by 1 of 10 founders, it will be fixed 5% of the time (surprise, surprise!). As the fitness of the homozygous phenotype decreases, its chances of being the winning allele decline. At a 5% reduction in fitness, 3.5-4% will still be fixed, most within 25 generations. At 15% the computer says the other allele will almost always win - if our slightly deleterious allele gets no boost from being linked to a selected gene or spread by a popular sire. However, one or both these conditions are usually violated, as discussed above. Furthermore, there is no guarantee that our selection will discriminate as finely as the computer.
If each such gene reduced fitness by only 5%, and the effects are additive, we could easily be facing a population with significantly lower litter sizes, shortened lifespans and greater susceptibility to non-genetic problems. Yet we would have no easily identifiable gene to pin it on.
Conclusions
Longevity and fertility, commonly regarded as indicators of "inbreeding depression", are reduced in canine populations which ave been inbred over a relatively short time period (Laikre and Ryman, 1991; Nordrum, 1994). However, most of the inbreeding in domestic dog populations does not appear to be due to breeders intentionally mating close relatives1 (though there are certainly exceptions), but to the loss of diversity due to drift and selection. The resultant loss of choices makes every individual a close relative, no matter what breeding strategy is employed.
The outcome for any breed will depend on both luck and on the breed's history. What is the effective population size? How many founders were there? Over how long a period prior to the closure of the stud books had the breed been refined? How intensive was the selection used to define type? Have there been any bottlenecks? How strong an influence have popular sires had?
What can we do?
1. We can control many of the obvious genetic diseases by supporting research aimed at locating the genes and developing direct DNA tests for the mutant alleles. Test results should be employed to make certain that carriers are only mated to clear individuals, rather than for wholesale elimination of carriers, which would further impoverish the gene pool.
2. We can explain to breeders that mutations will always be with us, and are not an indication of failure or bad breeding practice, and that an open exchange of information will produce the greatest rewards. We can also show them ways to achieve their personal goals without making choices that are detrimental to their breed.
3. We can attempt to educate breed clubs on the importance of maximizing diversity in the gene pool. As the keynote speaker at the recent AKC/CHF conference, Dr. Malcolm Willis, pointed out, few breeds even have a good idea of what their major genetic problems are, how many pups are in an average litter, or how long their dogs live. Fewer still have any idea of how to retain existing diversity or reduce the average inbreeding.
Notes:
1. Based on a study of 3 and 5-generation pedigrees of Australian Shepherds, Clumber Spaniels, Standard Poodles and Malamutes.
References
Brewer, G.M. (1999) DNA Studies in Doberman von Willebrand's Disease. Available online at: http://www.VetGen.com/vwdrpt.html
Garrod, A.E. (1902) The incidence of alkaptonuria: a study in chemical individuality. Lancet 2: 1616-1620. Available online at: http://www.esp.org/foundations/genetics/classical/ag-02.pdf
Laikre, L. and N. Ryman (1991) Inbreeding depression in a captive wolf (Canis lupus) population. Conservation Biology 5: 33-40.
Nash, J. (1990) "The Backcross Project" in The Dalmatian Quarterly, Fall 1990, Hoflin Publishing Ltd.
Nordrum, NMV (1994) Effect of inbreeding on reproductive performance in blue fox (Alopex lagopus). Acta Agriculturae Scandinavica, Sect. A, Animal Sci. 44: 214-221.
© John B. Armstrong, University of Ottawa, Oct. 1999
Selasa, 11 Oktober 2011
The Types Mutations
Section 8.1Mutations: Types and Causes
The
development and function of an organism is in large part controlled by
genes.
Mutations can lead to changes in the structure of an
encoded protein or to a
decrease or complete loss in its expression. Because a
change in the DNA sequence
affects all copies of the encoded protein, mutations can
be particularly damaging to
a cell or organism. In contrast, any alterations in the
sequences of RNA or protein
molecules that occur during their synthesis are less
serious because many copies of
each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
Many common human diseases, often
devastating in their effects, are due to mutations
in single genes. Genetic
diseases arise by spontaneous mutations in germ
cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell
anemia, which affects 1 in 500 individuals
of African descent, is
caused by a single missense mutation at codon 6 of
the β-globin gene;
as a result of this mutation, the glutamic acid at
position 6 in the normal
protein is changed to a valine in the mutant
protein. This alteration has a
profound effect on hemoglobin, the oxygen-carrier
protein of erythrocytes, which
consists of two α-globin and two β-globin subunits
(see Figure 3-11).
The deoxygenated form of the
mutant protein is insoluble in erythrocytes and
forms crystalline arrays. The
erythrocytes of affected individuals become rigid
and their transit through
capillaries is blocked, causing severe pain and
tissue damage. Because the
erythrocytes of heterozygous individuals are
resistant to the parasite causing
malaria, which is endemic in Africa, the mutant
allele has been maintained. It
is not that individuals of African descent are more
likely than others to
acquire a mutation causing the sickle-cell defect,
but rather the mutation has
been maintained in this population by interbreeding.
Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13).
In
a later section, we will see how normal copies of disease-related genes
can be
isolated and cloned.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
A
fundamental genetic difference between organisms is whether their cells
carry a
single set of chromosomes or two copies of each
chromosome. The former are
referred to as haploid; the latter, as diploid. Many simple unicellular organisms are
haploid,
whereas complex multicellular organisms (e.g., fruit
flies, mice, humans) are
diploid.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1).
Recessive
mutations inactivate the affected gene and lead to a loss of
function. For instance, recessive mutations
may remove part of or
all the gene from the chromosome, disrupt expression
of the gene, or alter the
structure of the encoded protein, thereby altering
its function. Conversely,
dominant mutations often lead to a gain of
function. For
example, dominant mutations may increase the
activity of a given gene product,
confer a new activity on the gene product, or lead
to its inappropriate spatial
and temporal expression. Dominant mutations,
however, may be associated with a
loss of function. In some cases, two copies of a
gene are required for normal
function, so that removing a single copy leads to
mutant phenotype. Such genes
are referred to as haplo-insufficient. In
other cases,
mutations in one allele may lead to a structural
change in the protein that
interferes with the function of the wild-type
protein encoded by the other
allele. These are referred to as dominant
negative
mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1).
Figure 8-1
For a recessive mutation to give
rise to a mutant phenotype in a
diploid organism, both alleles must
carry the mutation. However, one copy of a dominant mutant allele leads
to a mutant
phenotype. (more...)
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Recessive
and dominant mutations can be distinguished because they exhibit
different patterns of inheritance. To understand
why, we need to review the type
of cell division that gives rise to gametes (sperm and egg cells in higher plants and
animals). The body
(somatic) cells of most multicellular organisms
divide by mitosis (see Figure 1-10),
whereas the germ cells that give rise to gametes
undergo meiosis. Like body cells,
premeiotic germ cells are diploid, containing two of
each morphologic type of
chromosome. Because the two members of each such
pair of homologous chromosomes are descended
from different parents, their genes are similar but
not usually identical.
Single-celled organisms (e.g., the yeast S.
cerevisiae) that
are diploid at some phase of their life cycle also
undergo meiosis (see Figure 10-54).
Figure 8-2 depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Now,
let’s see what phenotypes are generated by mating of wild-type
individuals with mutants carrying either a dominant
or a recessive mutation. As
shown in Figure 8-3a, half the gametes
from an individual heterozygous for a dominant
mutation in a particular gene
will have the wild-type allele, and half will have
the mutant allele. Since
fertilization of female gametes by male gametes
occurs randomly, half the first
filial (F1) progeny resulting from the
cross between a normal
wild-type individual and a mutant individual
carrying a single dominant allele
will exhibit the mu-tant phenotype. In contrast, all
the gametes produced by a
mutant homozygous for a recessive mutation will
carry the mutant allele. Thus,
in a cross between a normal individual and one who
is homozygous for a recessive
mutation, none of the F1 progeny will
exhibit the mutant phenotype
(Figure 8-3b). However, one-fourth
of the progeny from parents both heterozygous for a
recessive mutation will show
the mutant phenotype.
Figure 8-2 depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Figure 8-2
Meiosis. A premeiotic germ cell
has two copies of each chromosome
(2n), one maternal and one
paternal.
Chromosomes are replicated during the S
phase, giving a
(more...)
Figure 8-3
Segregation patterns of dominant
and recessive mutations. Crosses between genotypically normal
individuals (blue) and mutants
(yellow) that are heterozygous for a
dominant mutation (a) or
(more...)
A
mutation involving a change in a single base pair, often called a point mutation, or a deletion of a
few base pairs generally affects the function of a
single gene (Figure 8-4a). Changes in a single base
pair may produce one of three types of mutation:
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b).
Figure 8-4
Different types of mutations.
(a) Point mutations, which involve alteration in a single base pair, and
small deletions generally directly affect
the function of only one gene.
A wild-type peptide (more...)
- Missense mutation, which results in a protein in which one amino acid is substituted for another
- Nonsense mutation, in which a stop codon replaces an amino acid codon, leading to premature termination of translation
- Frameshift mutation, which causes a change in the reading frame, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b).
Mutations
arise spontaneously at low frequency owing to the chemical instability
of purine and pyrimidine bases and to errors during
DNA replication. Natural
exposure of an organism to certain environmental
factors, such as ultraviolet
light and chemical carcinogens (e.g., aflatoxin B1),
also can cause
mutations.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5 illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs.
In
order to increase the frequency of mutation in experimental organisms,
researchers often treat them with high doses of
chemical mutagens or expose them
to ionizing radiation. Mutations arising in response
to such treatments are
referred to as induced mutations. Generally,
chemical mutagens
induce point mutations, whereas ionizing radiation
gives rise to large
chromosomal abnormalities.
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5 illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs.
Figure 8-5
One mechanism by which errors in
DNA replication produce
spontaneous mutations. The replication
of only one strand is shown; the other strand is
replicated normally, as shown at the
top. (more...)
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12.
Figure 8-6
Induction of point mutations by
ethylmethane sulfonate (EMS), a
commonly used mutagen. (a) EMS alkylates
guanine at the oxygen on position 6 of the purine
ring, forming O6-ethylguanine
(Et-G),
(more...)
Many common human diseases, often
devastating in their effects, are due to mutations
in single genes. Genetic
diseases arise by spontaneous mutations in germ
cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell
anemia, which affects 1 in 500 individuals
of African descent, is
caused by a single missense mutation at codon 6 of
the β-globin gene;
as a result of this mutation, the glutamic acid at
position 6 in the normal
protein is changed to a valine in the mutant
protein. This alteration has a
profound effect on hemoglobin, the oxygen-carrier
protein of erythrocytes, which
consists of two α-globin and two β-globin subunits
(see Figure 3-11).
The deoxygenated form of the
mutant protein is insoluble in erythrocytes and
forms crystalline arrays. The
erythrocytes of affected individuals become rigid
and their transit through
capillaries is blocked, causing severe pain and
tissue damage. Because the
erythrocytes of heterozygous individuals are
resistant to the parasite causing
malaria, which is endemic in Africa, the mutant
allele has been maintained. It
is not that individuals of African descent are more
likely than others to
acquire a mutation causing the sickle-cell defect,
but rather the mutation has
been maintained in this population by interbreeding.Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13).
Figure 8-7
Role of spontaneous somatic
mutation in retinoblastoma, a
childhood disease marked by retinal
tumors. Tumors arise from retinal cells that carry two mutant
Rb− alleles. (a)
In
(more...)
- Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry only one copy.
- Mutations are alterations in DNA sequences that result in changes in the structure of a gene. Both small and large DNA alterations can occur spontaneously. Treatment with ionizing radiation or various chemical agents increases the frequency of mutations.
- Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
- Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.
- In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells (Figure 8-2). The members of each pair of homologous chromosomes segregate independently during meiosis, leading to the random reassortment of maternal and paternal alleles in the gametes.
- Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses (see Figure 8-3).
By agreement with the publisher, this book is
accessible by the search feature, but cannot be browsed.
Copyright © 2000, W. H. Freeman and
Company.
and Causes
The
development and function of an organism is in large part controlled by
genes.
Mutations can lead to changes in the structure of an
encoded protein or to a
decrease or complete loss in its expression. Because a
change in the DNA sequence
affects all copies of the encoded protein, mutations can
be particularly damaging to
a cell or organism. In contrast, any alterations in the
sequences of RNA or protein
molecules that occur during their synthesis are less
serious because many copies of
each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
Many common human diseases, often
devastating in their effects, are due to mutations
in single genes. Genetic
diseases arise by spontaneous mutations in germ
cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell
anemia, which affects 1 in 500 individuals
of African descent, is
caused by a single missense mutation at codon 6 of
the β-globin gene;
as a result of this mutation, the glutamic acid at
position 6 in the normal
protein is changed to a valine in the mutant
protein. This alteration has a
profound effect on hemoglobin, the oxygen-carrier
protein of erythrocytes, which
consists of two α-globin and two β-globin subunits
(see Figure 3-11).
The deoxygenated form of the
mutant protein is insoluble in erythrocytes and
forms crystalline arrays. The
erythrocytes of affected individuals become rigid
and their transit through
capillaries is blocked, causing severe pain and
tissue damage. Because the
erythrocytes of heterozygous individuals are
resistant to the parasite causing
malaria, which is endemic in Africa, the mutant
allele has been maintained. It
is not that individuals of African descent are more
likely than others to
acquire a mutation causing the sickle-cell defect,
but rather the mutation has
been maintained in this population by interbreeding.
Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13).
In
a later section, we will see how normal copies of disease-related genes
can be
isolated and cloned.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly speaking, the entire set of genes carried by an individual is its genotype, whereas the function and physical appearance of an individual is referred to as its phenotype. However, the two terms commonly are used in a more restricted sense: genotype usually denotes whether an individual carries mutations in a single gene (or a small number of genes), and phenotype denotes the physical and functional consequences of that genotype.
A
fundamental genetic difference between organisms is whether their cells
carry a
single set of chromosomes or two copies of each
chromosome. The former are
referred to as haploid; the latter, as diploid. Many simple unicellular organisms are
haploid,
whereas complex multicellular organisms (e.g., fruit
flies, mice, humans) are
diploid.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1).
Recessive
mutations inactivate the affected gene and lead to a loss of
function. For instance, recessive mutations
may remove part of or
all the gene from the chromosome, disrupt expression
of the gene, or alter the
structure of the encoded protein, thereby altering
its function. Conversely,
dominant mutations often lead to a gain of
function. For
example, dominant mutations may increase the
activity of a given gene product,
confer a new activity on the gene product, or lead
to its inappropriate spatial
and temporal expression. Dominant mutations,
however, may be associated with a
loss of function. In some cases, two copies of a
gene are required for normal
function, so that removing a single copy leads to
mutant phenotype. Such genes
are referred to as haplo-insufficient. In
other cases,
mutations in one allele may lead to a structural
change in the protein that
interferes with the function of the wild-type
protein encoded by the other
allele. These are referred to as dominant
negative
mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is one in which both alleles must be mutant in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous individual carrying one mutant and one normal allele (Figure 8-1).
Figure 8-1
For a recessive mutation to give
rise to a mutant phenotype in a
diploid organism, both alleles must
carry the mutation. However, one copy of a dominant mutant allele leads
to a mutant
phenotype. (more...)
Some alleles can be associated with both a recessive and a dominant phenotype. For instance, fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies homozygous for this allele die during development. Thus the recessive phenotype associated with this allele is lethal, whereas the dominant phenotype is not.
Recessive
and dominant mutations can be distinguished because they exhibit
different patterns of inheritance. To understand
why, we need to review the type
of cell division that gives rise to gametes (sperm and egg cells in higher plants and
animals). The body
(somatic) cells of most multicellular organisms
divide by mitosis (see Figure 1-10),
whereas the germ cells that give rise to gametes
undergo meiosis. Like body cells,
premeiotic germ cells are diploid, containing two of
each morphologic type of
chromosome. Because the two members of each such
pair of homologous chromosomes are descended
from different parents, their genes are similar but
not usually identical.
Single-celled organisms (e.g., the yeast S.
cerevisiae) that
are diploid at some phase of their life cycle also
undergo meiosis (see Figure 10-54).
Figure 8-2 depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Now,
let’s see what phenotypes are generated by mating of wild-type
individuals with mutants carrying either a dominant
or a recessive mutation. As
shown in Figure 8-3a, half the gametes
from an individual heterozygous for a dominant
mutation in a particular gene
will have the wild-type allele, and half will have
the mutant allele. Since
fertilization of female gametes by male gametes
occurs randomly, half the first
filial (F1) progeny resulting from the
cross between a normal
wild-type individual and a mutant individual
carrying a single dominant allele
will exhibit the mu-tant phenotype. In contrast, all
the gametes produced by a
mutant homozygous for a recessive mutation will
carry the mutant allele. Thus,
in a cross between a normal individual and one who
is homozygous for a recessive
mutation, none of the F1 progeny will
exhibit the mutant phenotype
(Figure 8-3b). However, one-fourth
of the progeny from parents both heterozygous for a
recessive mutation will show
the mutant phenotype.
Figure 8-2 depicts the major events in meiosis. One round of DNA replication, which makes the cell 4n, is followed by two separate cell divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding germ cells with different mixes of paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n, where n is the haploid number of chromosomes. In the case of a single chromosome, as illustrated in Figure 8-2, meiosis gives rise to two types of gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Figure 8-2
Meiosis. A premeiotic germ cell
has two copies of each chromosome
(2n), one maternal and one
paternal.
Chromosomes are replicated during the S
phase, giving a
(more...)
Figure 8-3
Segregation patterns of dominant
and recessive mutations. Crosses between genotypically normal
individuals (blue) and mutants
(yellow) that are heterozygous for a
dominant mutation (a) or
(more...)
A
mutation involving a change in a single base pair, often called a point mutation, or a deletion of a
few base pairs generally affects the function of a
single gene (Figure 8-4a). Changes in a single base
pair may produce one of three types of mutation:
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b).
Figure 8-4
Different types of mutations.
(a) Point mutations, which involve alteration in a single base pair, and
small deletions generally directly affect
the function of only one gene.
A wild-type peptide (more...)
- Missense mutation, which results in a protein in which one amino acid is substituted for another
- Nonsense mutation, in which a stop codon replaces an amino acid codon, leading to premature termination of translation
- Frameshift mutation, which causes a change in the reading frame, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon
The second major type of mutation involves large-scale changes in chromosome structure and can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between nonhomologous chromosomes (Figure 8-4b).
Mutations
arise spontaneously at low frequency owing to the chemical instability
of purine and pyrimidine bases and to errors during
DNA replication. Natural
exposure of an organism to certain environmental
factors, such as ultraviolet
light and chemical carcinogens (e.g., aflatoxin B1),
also can cause
mutations.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5 illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs.
In
order to increase the frequency of mutation in experimental organisms,
researchers often treat them with high doses of
chemical mutagens or expose them
to ionizing radiation. Mutations arising in response
to such treatments are
referred to as induced mutations. Generally,
chemical mutagens
induce point mutations, whereas ionizing radiation
gives rise to large
chromosomal abnormalities.
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur. Figure 8-5 illustrates how one type of copying error can produce a mutation. In the example shown, the mutant DNA contains nine additional base pairs.
Figure 8-5
One mechanism by which errors in
DNA replication produce
spontaneous mutations. The replication
of only one strand is shown; the other strand is
replicated normally, as shown at the
top. (more...)
Ethylmethane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming O6-ethylguanine (Figure 8-6a). During subsequent DNA replication, O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C base pair is replaced with an A·T base pair (Figure 8-6b). The causes of mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in Chapter 12.
Figure 8-6
Induction of point mutations by
ethylmethane sulfonate (EMS), a
commonly used mutagen. (a) EMS alkylates
guanine at the oxygen on position 6 of the purine
ring, forming O6-ethylguanine
(Et-G),
(more...)
Many common human diseases, often
devastating in their effects, are due to mutations
in single genes. Genetic
diseases arise by spontaneous mutations in germ
cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell
anemia, which affects 1 in 500 individuals
of African descent, is
caused by a single missense mutation at codon 6 of
the β-globin gene;
as a result of this mutation, the glutamic acid at
position 6 in the normal
protein is changed to a valine in the mutant
protein. This alteration has a
profound effect on hemoglobin, the oxygen-carrier
protein of erythrocytes, which
consists of two α-globin and two β-globin subunits
(see Figure 3-11).
The deoxygenated form of the
mutant protein is insoluble in erythrocytes and
forms crystalline arrays. The
erythrocytes of affected individuals become rigid
and their transit through
capillaries is blocked, causing severe pain and
tissue damage. Because the
erythrocytes of heterozygous individuals are
resistant to the parasite causing
malaria, which is endemic in Africa, the mutant
allele has been maintained. It
is not that individuals of African descent are more
likely than others to
acquire a mutation causing the sickle-cell defect,
but rather the mutation has
been maintained in this population by interbreeding.Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including retinoblastoma, which is associated with retinal tumors in children (see Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele (Figure 8-7a). When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A second form of this disease, called sporadic retinoblastoma, results from two independent mutations disrupting both Rb alleles (Figure 8-7b). Since only one somatic mutation is required for tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic mutations. The Rb protein has been shown to play a critical role in controlling cell division (Chapter 13).
Figure 8-7
Role of spontaneous somatic
mutation in retinoblastoma, a
childhood disease marked by retinal
tumors. Tumors arise from retinal cells that carry two mutant
Rb− alleles. (a)
In
(more...)
- Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry only one copy.
- Mutations are alterations in DNA sequences that result in changes in the structure of a gene. Both small and large DNA alterations can occur spontaneously. Treatment with ionizing radiation or various chemical agents increases the frequency of mutations.
- Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
- Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.
- In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four haploid cells (Figure 8-2). The members of each pair of homologous chromosomes segregate independently during meiosis, leading to the random reassortment of maternal and paternal alleles in the gametes.
- Dominant and recessive mutations exhibit characteristic segregation patterns in genetic crosses (see Figure 8-3).
By agreement with the publisher, this book is
accessible by the search feature, but cannot be browsed.
Copyright © 2000, W. H. Freeman and
Company.
Langganan:
Komentar (Atom)