THE INHERITANCE OF LITTER SIZE, BODY WEIGHT, AND VARIABILITY, IN A CROSS BETWEEN TWO INBRED STRAINS OF MICE

Crosses were made between the two inbred lines, C57BL and BALB, and the effects of maternity and genotype on litter size, survival, 30- and 60-day weights, and their variances were assessed. The maternal effect of BALB reduced litter size by.78 to 1.91 when compared with reciprocal crosses of identical genotype. No maternal effect was found for C57; neither did the genotype of the young have any effect on the litter size borne by mothers of this strain. There was a significant increase (2.26 ±.67) in litter size when BALB females produced hybrid instead of inbred young. This was not the result of hybrid males being more prolific, because BALB males mated to non-BALB females produced the largest litters in this experiment. The cross F1 × F1 produced the largest mean litter size. A change in ration affected the litter size when BALB was the mother, but not when C57 was the mother. Fox chow plus linseed produced more young than any of the other three diets.There were no significant differences in the weights of males of any of the generations at 30 days, but the F1 females borne in C57 mothers were larger than the females of other generations, and it appears that the C57 female exerts a maternal effect which causes the female young to mature earlier. The data indicate that the mother exerts the greatest influence on 30-day weights, the genotype of the young having little or no effect. The F1 female has the greatest maternal effect: she increases the 30-day weight by 10–20%.At 60 days the F1 mouse was 15% larger than the corresponding inbred borne in the same female. The mean weights of the F1 and F2 were the same, but the backcrosses were 10% lighter.The variance at 30 days was larger than the corresponding variance at 60 days, and 90% of the variance was in the between-litters component. At 30 days the largest variances were found in the P1, F1, and B.C. generations, while the smallest was found in the F2 indicating that much of the variability was the result of the maternal environment. At 60 days the variances for all generations were essentially the same.MacArthur's large and small strains, which had been produced by 30 generations of selection with only moderate inbreeding, were carried through 20 generations of brother × sister mating. This inbreeding brought about a decrease in the weight of the large line, and an increase in the weights of the small strain. The former may be attributed to the loss of the heterozygotic effect on size, while the latter may be the result of natural counterselection; the smallest mice had few or no young.