Abstract
Hybrid vigor and variability in F1 hybrid on the traits of the mammary growth response to estrogen and the body weight in mice were studied on 4 inbred strains and F1 hybrid between them. Strains bc and C (undergone full-sib mating for mcre than 25 generations) and KA and KB mice (undergone full-sib mation gor 12 generations), which have been selected for mammary growth response, were used in this study. All tested mice were recieved total dose of 12.5 I.U. of synthetic estrogen from 20 days of age. They were killed at 35 days and their third thoracic mammary glands were removed from the skin to prepare them by the whole mount method. The mammary growth response was expressed by the mammary duct area. Body weight were measured at birth, 12 days, 20 days and 35 days of age. In these characters, the potence ration ((F1-Pm)/|P-Pm|) of F1 hybrid, which indicated an effect of heterosis, were calculated. The presence of maternal influence was examined. Variations of body weight and mammary growth response were compared between inbred strains and F1 hybrid between them. The results obtained are briefly summarized in folllowing: (1)The potence rations of F1 hybrids were more than 1 in moset cases, and accordingly high heterosis in these characters seemde to be present in F1 hybrids. On the body weight, the values were the smallest at 12 days and thereafter increased gradually with advancing stages. (2)In mammary growth response the potence ratio of F1 hybrid between unselected strain × sekected was the smallest in both sexes. But the value of mammary growth response of selected × selected was the highest, and that of selected × unselected was next and that of unselected × unselected was the lowest. (3)Maternal influence on body weight was significant at 5% level at 35 days of age only. Proportion of the magnitude to total variance was considerably large at birth, and thereafter decrease with advancing stages. That of mammary growth response was significant at 5% level in female but not in male. (4)In body weight at any stage and mammary growth resoinse, F1 hybrid proved to be more unifrom than inbred strains. (5)Cariability of mammary growth response in F1 hybrid of selected × selected was the most unifrom, that of unselected × was next and that of unselected × unselected was the largest in both sexes. (6)Close relationship was seen between mammary growth response and body weight at 35 days of age within each inbred strain or F1 hybreid, but no relationship was seen inter inbred strain groups and inter F1 hybred proups in both sexes. A suggestion with regard to obtaining animals with the smaller phenotypic variablity for a given trait, was put forward.
