Response to Selection

Response to Selection

1. Response to Selection Topic : Response to selection, S election Differential , Selection Intensity Dr Syed Shanaz

2. RESPONSE TO  SELECTION The change produced by selection is the change of population mean in the offspring, symbolized by “R”: Response to selection is the difference of mean phenotypic value between the offspring of the selected parents and whole of the parental generation before selection Response to selection is also called as the expected genetic gain, symbolized by   G . R or   G = h 2   S where, h 2  = heritability S = selection differential R or    G/year = h 2  S / GI where, h 2  = heritability S = selection differential GI = generation interval

3. FACTORS AFFECTING IT Generation interval :   It is the time interval between generations and is defined as the average age of the parents when the offspring is born. Varies between species and selection procedure Management practices for early breeding in females reduces GI and breeding practices like progeny testing increases the GI The average generation intervals for different species are: Species Generation Interval (in years) Males Females Average Dairy cattle 3 - 4 4.5 - 6.0 4 - 5 Beef cattle 3 - 4 4.5 - 6.0 4 - 5 Sheep 2 - 3 4.0 - 4.5 3 - 4 Swine 1.5 - 2 1.5 - 2.0 1.5 - 2.0 Chicken 1 - 1.5 1 - 1.5 1.0 - 1.5 Horse 8 - 12 8 - 12 8 - 12

4. A ccuracy for selection is directly related to the heritability of the trait If heritability is high, the selection on phenotype will permit an average estimation of breeding value If heritability is low, many errors are possible. Increased accuracy in selection can be obtained by comparing the animals in controlled environmental conditions Corrections may be made for the age of the individual, age of the dam and sex to remove non-genetic variations The techniques may increase the heritability of the trait by reducing the environmental variation Accuracy can be increased by using additional measurements for the trait from the same individual using measurements of correlated traits and using measurements of relatives . Accuracy of selection :  

5. Selection limit When selection is continuous, the response to selection will be more for a few generations, then it slows down and finally stops When the response to selection has stopped, the population is said to be at “plateau” or “selection limit”. Main cause for this is fixation of favourable genes.  reduction or absence of genetic variation Further improvement depends on introduction of new genetic variation New genetic variation can be introduced by cross breeding, mutation and genetic engineering.

6. FACTORS AFFECTING GENETIC GAIN   The factors affecting the response to selection are heritability, selection differential and generation interval Maximum gain will result when the selection differential (S) and the heritability ( h 2 ) is high and the Generation Interval is low Heritability:  G enetic gain depends on the h 2  of the character in generation from which the parents are selected and if the h 2  is high, genetic gain will also be more, because the environmental variation will be less Selection differential:  The average superiority of the selected parents is called as selection differential, symbolized by “S ” It is the difference between the mean phenotypic value of the individuals selected as parents and the mean phenotypic value of all the individuals in the parental generation before selection

7. S = (Ps - P) where Ps = mean of the selected parents P = mean of the population S = i  s p where i = intensity of the selection s p  = phenotypic standard deviation The selection differential may also be expressed in terms of phenotypic standard deviation  standard deviation is the measure of variability

8. Intensity of the selection Also called as selection pressure and it is the mean deviation of the selected individuals in units of standard deviation The intensity of selection is symbolized by “i ” It depends on the proportion of the individuals selected i = Selection differential / Phenotypic standard deviation

9. FACTORS AFFECTING  SELECTION DIFFERENTIAL   Proportions of the animal selected for bree ding Smaller the number larger the selection differential herd size L arger the herd size, smaller the proportions of animals selected Reproductive rate Selection differential will be less in cattle; more in pigs because of litter size Use of artificial insemination and frozen semen increases selection differential or selection intensity in case of males and in females Super ovulation and embryo transfer increases the selection differential or selection intensity

10. Species Percentage of animals to be selected Females Males Dairy cattle 4 - 5 50 - 60 Beef cattle 4 - 5 40 - 50 Sheep 2 - 4 45 - 55 Swine 1 - 2 10 - 15 Chicken 1 - 2 10 - 15 Horse 2 - 4 40 - 50 Percentage of males and females to be selected for breeding to maintain a constant herd size for different species

11. Thanks