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SummaryNinety-five calves, mostly Angus, but also of Hevefovd and Shovthovn breeding, born in the University of California herd, were studied, including 54 , born during Spring 1970 , genotypically distributed as 7 mm, 33 m-!-and 14 + + (m being the symbol for the autosomal allele for doublemuscling).Conformation was evaluated at yearling age by scoring superficial muscles for bulginess and the lateral shape of the rump, and combining the scores into an index (h). Differences between mm and m + and between m-! and -!-!-, were significant (P ! o.ooi). Comparing m+ with !-+, the expression of the m gene was enhanced in bulls; the interaction between genotype and sex was significant (P < 0 . 005 ).The phenotypic variance of heterozygotes (m-!) exceeded that of normals (+ +) for 1 2 (P < 0 . 05 ) and 3 65-day liveweight (P < 0 .05), height at withers (o.i ! P < 0 . 25 ) and cannon bone (metacarpus) circumference (P ! 0 . 25 ).For each of these four traits, mtM—m-)-, the absolute difference between means, was more than twice as great as I m + -+ + I. -The effect of the m gene in the heterozygote on the mean and standard deviation, was greater for muscle conformation (1!) and liveweight than for skeletal traits. The

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