Methods of selection in animal genetics and breeding
- 1. METHODS OF SELECTION Presented by: Dr. Jayesh Vyas
- 2. Methods of Selection 1.Single trait 2.Multiple trait selection Success of rising animal depends on improvement in more than one trait. Single trait selection can have unexpected and undesirable consequence due to genetic correlation and environment response.
- 3. • Methods of selection- Simultaneous selection for many traits can be applied based on individuals own performance by adopting any of the procedure of selection. One may wish to adopt tandem selection or ICL methods or one may evaluate the individuals on the value for each of the traits selected for and then sum of these values to give a total value for all the traits. The animal with the highest score is then selected. These procedure are known as methods of selection
- 4. 1.Tandem Selection In this method of individual selection the selection is practiced for one trait only at a time until a satisfactory improvement is achieved . The efficiency of this selection depends on the genetic correlation among the traits under selection. Limitations – 1. This method is less efficient than other methods. 2. It requires more time for improvement in all the traits
- 5. 2.Independed culling levels (ICL) This method of individual selection involves selection for two or more traits at a time. A minimum standard for each trait is fixed and every animal to be selected must meet the minimum standard fixed for any one of the trait will be rejected, irrespective of the fact that how good the animal was in other trait.
- 6. • Advantages of ICL 1.Selection for more than one trait. 2. Culling at early stage. Disadvantage of ICL 1.No compensations for other traits 2. Culling level- tedious job to decide CL 3.More emphasis to early expressed traits 4. Selection intensity
- 7. • ICL method of selection is used in selection of cattle for show purpose. • The animal is selected with excellence of type, colour and conformation traits ignoring its performance for economic traits. Selection intensity under ICL:- . The selection intensity is decreased by selection for more than one trait. . The genetic correlations among traits also influences the selection intensity. selection intensity= 1/√n n= no traits which are equally important and independent
- 8. • 3Total score or selection index .In this method of mass selection , the selection is practiced for several traits simultaneously. But this method differ from ICL in that .The culling level are flexible. . .Each trait is weighted by a scores for individual traits are summed to a total score (index value) for each animal. .Superiority in some trait can make up deficiency in other, unlike ICL which discards an animal; failing to qualify in one trait regardless of its superiority in other trait.
- 9. Efficiency of S.I. over other methods of selection S.I. is a more efficient method of selection than others (1) It allows the individuals which are superior in some traits to be selected regardless of their inferiority in other traits . (2)The advantage of index method increase with the no. of trait, thus it requires less time to bring about an overall improvement
- 10. Use of discriminate function to construct selection index • Selection index method involves the technique of discriminate function originally developed by Fisher (1936) and used by Smith (1936). The index will take the form as: I=b1x1+b2x2+.........+bnxn Where, xi =represent the phenotypic value for the different traits bi=the weighting factors given to each the trait.
- 11. Genetic basis of selection index:- The genetic worth of an individual is expressed as H=a1G1+a2G2+a3G3+.........anGn where, G1, G2.....Gn are the genotypic value of individual character. a1, a2..........an to their relative economic importance. H=The overall net genetic worth for several traits
- 12. .The phenotype of an individual based on the phenotype performance of various traits is defined as: I=b1x1+b2x2+.........+bnxn .Where X1,X2....Xn=the phenotypic value of the trait. b1, b2.....bn=the coefficient of each trait. .These coefficient are to be estimated in such a way that the correlation b/w H and I become maximum.
- 13. Information required for construction of selection index- • The construction of a selection index requires:- 1.The economic value of each of the trait. 2.The genetic and phenotypic variance of all traits. 3.The genotypic and phenotypic co-variance among traits.
- 14. 1.Estimation of economic value:- . It is important to know the method of estimating the relative economic value of the trait, by taking the example of cattle. . The most important economic traits are age at first calving (AFC), Milk yield, lactation length, peak yield, and milk yield per day of calving interval etc. . In order to bring about similarity among different traits, the economic value of a standard unit of each trait is considered.
- 15. Construction of selection index:- selection index is considered in following steps- 1.Estimation of genetic and economic parameters- The following genetic and economic parameters of the traits included in selection for improvement are estimated:- I. The economic value of for all the traits based on price data and profit. II. The genetic and phenotypic variance of all traits. III. The genotypic and phenotypic co-variance among traits.
- 16. 2.Estimation of partial regression coefficients:- The selection index (I)is: I=∑b i=b1x1+b2x2+.........+bnxn • Where Xi=the phenotypic value of the trait. bi=are the partial regression coefficient of each trait.
- 17. • The regression coefficients are estimated by setting and solving the normal simultaneous equations for n traits under selection as follows:- 1 . bi Vpi+b2 CovP12+...+bn Cov P1n=a1 VA1+a2 Cov A12+....+an Cov A1n . 2. bi Cov P21+b2 Vp2+...+bn Cov P2n=a1CovA21+a2 VA2+....+an CovA2n 3. bi Cov Pn1+b2 Cov Pn2+...+bn Vpn =a1CovAn1+a2CovAn2+....+bn Van .
- 18. • The above equations are solved for bi values. The variances and covariance and economic values are first determined and put into the above equations to solve for bi values. Selection index:- The selection index I=b1x1+b2x2+.........+bnxn the bi and xi values are put up in the index to obtain the index values for each animal and the animals with top highest index values are selected.
- 19. Practical problems in constructing selection index:- 1.The information required is not available or accurate enough. 2.The relative economic importance of traits varies from time to time and in different locality. 3.It requires information of price and cost of production, of the traits, but sometime the economic values changes due to change in market demand. 4.To find the genetic and phenotypic parameters is not an easy job when the no of traits exceeds three or more.
- 20. Comparative efficiency of different methods of selection:- • Hazel and Lush (1942) compared the efficiency of these three methods in terms of genetic gain, assuming that traits under selection were independent, and had equal heritability, economic weight and variance. (1)In tandem method the selection may be for unimportant character in the beginning of first few generations and selection for more important character may be for a brief period.
- 21. (2)In ICL method ,the culling levels may be set too high for one trait and too low for other. (3)In selection index method there may be mistake in estimating genetic parameters and economic value. . Lastly, the selection progress is also affected by animal health which affects p (proportion of selected individuals),and also by the breeders ability to record the observations properly. .Thus ,Hazel and Lush (1942) concluded that index method was most superior and tandem selection least, whereas the efficiency of ICL method was intermediate.
- 22. • Two trait selection index:- When two trait for construction of selection index are used then the genetic worth of an individual can be expressed as H=a1G1+a2G2 where- G1,G2 = genotypic values of individual character a1a2= refer as their relative economic importance.
- 23. • The phenotype of an individual based on the phenotypic performance is as: I=b1X1+b2X2 Where- X1X2 are the phenotypic values of the traits 1and 2 b1b2 are coefficient of each trait. These coefficient are to be estimated in such a way that the correlation b/w H and I become maximum.
- 24. Thank You