Methods of selection
- 1. TOPIC : METHODS OF SELECTION DR SYED SHANAZ Division of AGB
- 2. METHODS OF SELECTION TANDEM METHOD • Breeder selects & improves only one trait at a time until it reaches an acceptable level, then shifts to another and so on for a third • Efficiency of this method is dependent on the genetic relationships /correlation among the traits • Traits which are favourably or positively correlated, selection for the first trait will also automatically improve the other trait and vice versa • It is easy an method
- 3. Disadvantages • If not related genetically, whatever achievement is made in the first trait is lost when attention is directed to another trait • Highly inefficient method unless the traits selected are genetically positively related • If there is a positive correlation, then the results may be desirable in the other trait also • For a negative correlation, the efforts will be undesirable • The rate of net improvement becomes very small Since a very long period is involved in the selection practiced, the breeder might change his goals too often or become discouraged and not practice selection effectively
- 4. • Here selection may be practiced for two or more traits at a time or simultaneously • Minimum standard (culling level) is set, for each trait , every animal must meet the minimum standards to be selected for breeding purposes • Failure to meet the minimum standard for any one trait makes the animal to be rejected • May lead to culling of some genetically very superior animal • Properties selected for each trait will depend up on the total number of animals screened for the breeding INDEPENDENT CULLING METHOD
- 5. • This method reduces selection intensity of the traits to be selected • The negative correlation among the traits will make further reduction in selection intensity • Selection based on independent culling method is easy to perform but becomes complicated when more traits are considered and if there is negative correlation between traits Only few important traits should be considered in this method Disadvantages
- 6. • Most effective method of selection • Selection index is a single numerical value within the total scores given for each trait considered in the selection • Each trait is weighted, by giving score and an individual trait score is summed up to the total score for the each animal within the selection criteria • Individuals are ranked and selected on the basis of their selection indices SELECTION INDEX OR INDEX SELECTION OR TOTAL SCORE METHOD
- 7. Total score or an Index • Individual specification for a number of traits can vary greatly and is combined into one value for the animal Total score or an Index • The high merit in one trait can certainly be used to compensate the deficiencies in other traits • An index is simply a means of putting a whole lot of different information into one value • The information and the score should be fixed, based on – Variation seen in each trait – the phenotypic standard deviation – Heritability of the traits – Phenotypic and genetic relationships (correlation) between the traits – Relative economic value of the traits
- 8. • Aim in computing an index is to give the best prediction of the animal’s breeding value i.e. what it will produce when the animal breeds • If one component is missing then benefit can be obtained by predicting the missing one from the others that are present • Index selection is predicted to be n times as efficient as independent culling levels where n is the number of traits involved • The greater the number of traits involved, the index becomes more reliable than the independent culling method ADVANTAGES
- 9. • Higher economic value should be given to milk production and correspondingly lower economic value to the reproductive efficiency in dairy cattle I = b1X1 + b2X2 + b3X3 + …….. + bnXn Where, I – Index value or genetic prediction n – Number of traits of information b1 to bn – coefficients obtained based on the relative importance of heritability of each trait and genetic relationships of the traits concerned X1 to Xn – Measurement of each of the traits incorporated (phenotypic values Considerations
- 10. • Index selection has been more widely used with sheep and swine than in beef and dairy cattle • Large volume of accurate data of population is necessary to provide information to compute the selection index. • Indices computed from inadequate or erroneous information can be ineffective in selection • A trait that is highly heritable can be given adequate weightage than one with low heritability conclusion • Selection index is a total score that includes all the advantages and disadvantages of an animal for those traits considered for selection • The amount of weightage given to each trait depends on their relative economic value, heritability of the character and genetic correlation between characters • A trait, which is highly heritable, can be given greater score than a trait, which has a low heritability
- 11. • Most efficient (best method) among the three (Tandem, Independent culling and Selection Index) methods because it results in better genetic improvement • The index is the best estimate of an animal’s breeding value • The only disadvantage is the traits vary in importance from time to time and the index built at one time will not be applicable for all times. • Needs to be constructed and modified from time to time
- 12. Use of discriminant function to construct an index • DF as a basis for making selection is aimed at discriminating the desirable genotypes from undesirable ones on the basis of their phenotypic performance • The index combines different traits in a way to maximize probability of progress in aggregate economic value Genetic Basis of S.I • The DF or Selection index are obtained by fitting a multiple regression equation for predicting a dependant variable from two or more known independent variables • The observed phenotypic variation in the trait representing differences in genetic makeup that are heritable should be known Phenotypic value (x) X=G+E
- 13. The overall net genetic improvement (H) for several traits is sum of genetic gains made for several trait i.e n H = Gi i=1 The aggregate genetic worth (H) of individual or average genetic superiority of a selected group as per Hazel and Lush (1942) H = a1G1 +a2G2+………+anGn = aiGi ai= relative economic values which measures amount by which profit is expected to increase for each unit change in Xi Gi are the expected values of Xi due to additive gene effects When the equations are written in matrix form we get The genetic worth or BV is given by discriminant function or Index I Index (I ) should be closely related to composite breeding value (H) When the equations are written in matrix form we get bP=aG , therefore b=P-1 a G