Sex-linked and maternal effects on growth in chickens.

A study was designed to estimate the contribution of various types of genetic variation in growth of chickens using populations that widely differed in growth rate. Populations tested included a commercially developed sire line (CM), a fertility-selected, broiler-type line (FS), and the pureline Jersey Giant (JG). A full set of F1 crosses and purelines of both sexes was used to estimate the following genetic effects: heterosis due to the autosomes, heterosis due to the sex chromosomes, average line effects, maternal effects, and additive sex linkage. Average line effects were highly significant beyond hatching for both body weight and shank length, with rank-order CM greater than FS, which was greater than JG. Significant differences between the sexes within a line were observed for CM and JG line effects, confirming the existence of sex-linked factors. There was no average heterosis or sex-by-heterosis interaction for growth-related traits, but specific heterosis was frequently significant. Maternal effects were sporadic and generally of little importance beyond hatching. Additive sex linkage, however, was highly significant and influenced body weight beyond hatching. These data indicate that the primary advantages in increasing growth via cross-breeding in chickens comes from average line effects and additive sex linkage. Heterosis and maternal effects can influence performance, especially in specific crosses, but these effects appear to be sporadic and of less importance than sex linkage.