Biological detection of heterozygosity for double muscling in cattle. I. Terminal innervation ratio.

The extent of axonal branching of peripheral motor nerves in musculature of homozygous and heterozygous double-muscled cattle and of nondouble-muscled cattle was investigated. The purpose was to determine whether there are differences between genotypes and if such differences could be useful as objective criteria for identifying heterozygotes. Significant differences were found between the functional terminal innervation ratios for double-muscled and nondouble-muscled groups for both the semitendinosus and semimembranosus muscles. However, there was considerable overlap among individual animals of different genotypes. Because of this overlap, and the possibility that axonal branching may be related to the degree of muscle enlargement, we concluded that terminal innervation ratios would be of marginal value in identifying double-muscled heterozygotes.