HOMOGAMETIC HETEROSIS IN CROSSBREEDING EXPERIMENTS WITH SHEEP

Data from two crossbreeding experiments with sheep were analyzed to test the difference in heterosis between males and females in the F 1 , backcross, and three-breed crosses. The characters studied were birth and weaning weights and preweaning daily gain. In the F 1 no marked differences in heterosis in weaning weight and preweaning gain were observed between sexes while in birth weight the two experiments gave conflicting results. In backcrosses and the three-breed crosses, females showed higher heterosis than males in the three characters studied. The ratios of heterosis in males to that in females did not support the hypothesis that differences between sexes in heterosis are due to the action of the sex chromosomes. The results of the two experiments suggested a possible interaction between the X and Y chromosomes of different breeds. The results in general did not give good evidence of homogametic heterosis in crossbreeding experiments with sheep.