Fertility and growth of mice with the rat growth hormone (rGH) transgene were examined using a strain of mice where the effect of the rGH transgene on body weight was apparent in adults. In 338 progeny of male parents hemizygous for the rGH transgene and female parents lacking the transgene, there was a highly significant (P < 0.01) deficiency of progeny with the rGH transgene from what was expected from the Mendelian segregation. The fertility rate of 58.7% to 60.0% when one or both parents carried the rGH transgene was highly significantly lower (P < 0.01) than the 94.6 % rate when neither parent had the rGH transgene. The average body weight of individual progeny was heaviest when both parents were hemizygous, followed by that for mating groups where only one of the parents was hemizygous and the mating group where neither parent had the rGH transgene. The ranking of the mating groups for average body weight of individual progeny was consistent at 21, 42 and 63 days. The production index defined as the product of the fertility rate of female breeders and average body weight of their litters at 63 days was lower in the mating groups where the female parent was hemizygous for the rGH transgene than in the groups where female parents lacked the transgene. Mice with the rGH transgene, although heavier at all ages, had a lower fitness than those without the transgene and their overall productivity was lower than that of mice lacking the rGH transgene. These results suggest that desirable effects of gene transfer may be accompanied by reduction of performance of transgenic animals in other (economically) important traits.
