Evidence for a major gene for rapid postweaning growth in mice

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Abstract
SUMMARY: Mice gaining 3 or more standard deviations above the mean were noted beginning in generation 25 in a line selected for high 21–42 day weight gain. The exceptional growth rate appears to be due to an autosomal recessive gene, based on the following: (1) the exceptional individuals appeared suddenly, in only one of 2 closely related sublines; (2) mating high growth individuals to unrelated, normal size strains produces relatively uniform F1's with mean gains below the mid-parent average; (3) F2's have a distribution markedly skewed towards high gain and a coefficient of variation approximately double that of F1's; (4) true breeding high growth strains can be established in one generation by intermating the largest F2's; (5) intermating normal F2's produces progenies with a distribution similar to the F1except for a few large segregates; (6) high growth segregates have been obtained in F2's from each of 4 successive backcrosses to the C57BL/6 inbred line. The symbolhg(high growth) is proposed for the postulated gene, which appears to be completely recessive. Frequency of positively identified segregates in F2's and backcrosses is on average less than 25 and 50%, due probably to some overlap ofHg- andhghgdistributions. Gain ofhghgindividuals from 21–42 days is 30–50% higher than ofHg- contemporaries; mature weight is also much higher, while 21-day weight ofhghgindividuals in segregating litters is slightly lower. Fertility of homozygotes ranges from normal to as much as 40% lower than for comparableHg- mice;hghgmice are not obese. The gene may provide a useful model for study of regulation of mammalian growth.