Selection for Breast Angle at Eight Weeks of Age

THE magnitude of the response obtained from artificial selection depends on the additive genetic variance (heritability in the narrow sense) of the trait and the amount of selection pressure. A characteristic for which there has been no previous selection usually responds uniformly for several generations to individual phenotypic or mass selection. Thus, the expected genetic change in the next generation can be estimated from the product of the heritability of the character and the difference between the means of the selected parents and that of the generation from which they came. The discrepancy between the observed and expected response to mass selection is generally small enough to be explained by errors in sampling. When the discrepancy is fairly large, however, other explanations are necessary. These could be either, (1) a lack of additive gene effects, (2) a preponderance of non-additive gene effects or (3) natural selection cancelling the effectiveness of …