Forces_Changing_Gene__ genetics and breeding.pptx

Forces_Changing_Gene

1. Online Class ANIMAL GENETICS & BREEDING UNIT – II Principles of Animal & Population Genetics Lecture – 2 Forces changing gene and genotype frequency Dr K G Mandal Department of Animal Genetics & Breeding Bihar Veterinary College, Patna Bihar Animal Sciences University, Patna

2. Forces changing gene and genotype frequency

3. Fig. A large random mating population m = proportion of immigrants (1- m) = proportion of natives q m = frequency of a certain gene (A 2 ) at a given locus in immigrants. q o = frequency of the same gene (A 2 ) among the natives. New gene frequency in the whole population, q 1 = mq m + (1-m) q o Change of gene frequency due to immigration: ∆q = q1 – qo = mq m + (1- m) qo – qo = mq m + qo – mqo - qo = m( q m - qo ) Change of gene frequency due to migration m q m (1-m) q o Natives Immigrants

4. Conclusion: the rate of change of gene frequency in a population subject to immigration depends on : i . Immigration rate ii. Difference of gene frequency between immigrants & natives.

5. Change of gene frequency due to mutation Considering a locus with two alleles A 1 & A 2 Initial frequency of A 1 = p o , A 2 = q o Suppose A 1 mutates to A 2 @ u per generation & A 2 mutates to A 1 @ v per generation. Thus, gain of A 2 gene = up o & loss of A 2 = vq o Amount of change in gene frequency in one generation of mutation, ∆q = up o - vq o = Gain - Loss u v A 1 A 2

6. The relative magnitude of gain and loss per generation will decide the magnitude of q ( A 2 ). At equilibrium, gain and loss will be equal . At equilibrium, change of gene frequency (∆ q) will be equal to zero. ∆ q = up – vq = o or u (1- q) – vq = o u – uq = vq u = uq + vq u = q(u + v ) or q = u / (u + v )

7. Conclusion At equilibrium u and v are constant which makes the value of q as stable. 2. The magnitude of q S(A 2 ) at equilibrium is independent of initial gene frequency in the population but it is determined entirely by the relative magnitudes of u & v. 3. Mutation rates are very low, 10 -4 to 10 -6 per generation. 4. Reverse mutation is one-tenth to that of forward mutation.

8. Change of gene frequency due to selection According to I.M. Lerner: Selection is the non-random differential reproduction of genotypes. According to J.L. Lush: Selection is the difference in reproduction rates within a population whereby some individuals tend to have more offspring than the other animals. Fitness:- Contribution of offspring to the next generation is known as fitness of an individual. It is also known as adaptive value or selective value . 2 . Coefficient of selection :- Proportionate reduction in gametic contribution of a particular genotype in comparison to the standard genotype, usually the most forward genotype. It is denoted as ‘s ’. It is Intensity of selection. 3. If coefficient of selection ‘s’ of an individual is 0.1, then its fitness = 1 – 0.1 = 0.9 or 90%.

9. Degree of dominance: Dominance with respect to fitness. Different degrees of dominance considering a locus with two alleles A 1 and A 2 : No dominance A 2 A 2 A 1 A 2 A 1 A 1 1-S 1-½ s 1 Complete dominance A 2 A 2 A 1 A 1 , A 1 A 2 1-s 1 Over dominance A 2 A 2 A 1 A 1 A 1 A 2 1-s 2 1- s 1 1

10. Change of gene frequency due to selection under complete dominance and selection operates against a recessive genotypes. Total gametic contribution = 1-sq 2 Change of gene frequency , q = sq 2 (1-q)/1-sq 2 Conclusion: The effect of selection on gene frequency depends not only on the intensity of selection, “s” but also on the initial gene frequency.   Particulars Genotype A1A1 A1A2 A2A2 Total Initial frequencies p 2 2pq q 2 1 Coefficient of selection s Fitness 1 1 1-s Gametic contribution p 2 2pq q 2 (1-s) 1-sq 2

11. Dispersive Process: The process by which the change of gene frequency in a small population can be predicted only in amount but not the direction. Random drift: The random change of gene frequency due to sampling is known as random drift. Genetic drift: Random drift is also known as genetic drift. 3. The change of gene frequency in a small population is erratic in manner from generation to generation with no tendency to revert to its original value. For this reason, the random drift is known as genetic drift.