Breeding methods for cattle, pigs, and poultry in the United States : including inter-state co-operation project no. 253.

  • European Productivity Agency.
Date:
[1957]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Breeding methods for cattle, pigs, and poultry in the United States : including inter-state co-operation project no. 253. Source: Wellcome Collection.

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    genes forming their respective genotypes. Each gene in a particular population has an average additive effect, either plus or minus, in genotypic combination with the other genes. Thus each individual genotype would have an expected value derived by simply adding all the average effects of its constituent genes. Variation among these expected values constitutes the additive genetic variance. 36. It may be mentioned that the ratio of estimated additive genetic variance in a population to the actually observed vari¬ ance in the same population is heritability (Н2) in the narrow sense. 37. Most of the breeding plans to date have been designed on the basis of the heritability in the narrow sense of the charac¬ ters involved. If heritability of the character is relatively high, individual selection would be emphasised with slight, if any, consideration to family merit. On the other hand, low herit¬ ability would bring a greater emphasis on family merit and pro¬ geny test. There appears to be little doubt that the conventional breeding methods on the basis of individual and family merit would be superior to other breeding methods in changing the frequencies of genes responsible for additive genetic variation . However, if the additive genetic variance is small, ways and means of exploiting other types of hereditary variation become increasingly important. Epistatic genetic variance 38. This fraction is due to non-additive effects between non¬ allelic genes in genotypic combinations. Such effects occur when two or more non-allelic genes interact to produce a specific effect which is not the cumulative or additive action of the interacting genes. 39. Evidence has been presented indicating that a multitude of interactions must be involved in the inheritance of quantitative characters. The relative magnitude of the epistatic variation or gene effects as contrasted to the additive genetic and dominance effects is largely a matter of conjecture. It probably varies in importance from one characteristic to another. 40. The consequences of epistatic genetic variance are mainly that if such interactions are widespread and involve several loci, then neither individual or mass selection nor family selec¬ tion will be very effective in improving the population. Instead, the formation of inbred lines, endeavouring to segregate out de¬ sirable homozygous epistatic combinations, would be desirable. The superior inbred lines can be recombined for further selec¬ tion or they can be maintained as inbred lines for hybridization. Likewise, reciprocal and recurrent selection provide an oppor¬ tunity for increasing the frequency of those genes which would combine for superior epistatic combinations in the cross 16