Direct and maternal effects of egg-size genes on hatchability.

A N I M A L breeders often select the extreme individuals in a population. This may conflict with natural selection for reproductive fitness. For example, chickens with intermediate body weights and egg weights usually have the highest hatchability. LATTER and ROBERTSON ( 1962) hypothesized that the decline in fitness with selection for a metric trait is a consequence of gene frequency changes of two kinds: one results from directional selection and the other from random fixation of genes due to the restricted size of the selected population, i.e., from inbreeding depression. The first objective of our study was to compare the relative effects of inbreeding depression and egg size on hatchability in small populations undergoing directional selection for egg weight. The second was to investigate the path by which egg size genes influence hatchability-whether as a direct effect on hatchability or as an indirect (materna1)effect.