Red to White Fiber Ratios as an index of Double Muscling in Beef Cattle2

From the available information, it appears that the homozygous DM animal can be identified on the basis of muscle fiber characteristics. In the three muscles studied by Holmes and Ashmore, the homozygous DM animal had larger white fibers, more white fibers and thus a larger percentage of the area classified as white. Of particular interest is the lack of β red fibers in biopsy samples of the cutaneous truncii of homozygous DM animals reported by Holmes and Ashmore (1972). In the semimembranosus and longissimus muscles of the homozygous mutant, the microscopic appearance is distinctively different from that of the other genotypes. The classification of the homozygous DM animal should not be the important factor in this discussion since the phenotypic characteristics of this genotype are, except in a few instances, very distinctive. However, the identification of the heterozygote to preclude its classification as a normal animal is more important since phenotypically, such animals can be similar in phenotypes to either or, in some cases, neither of the two homozygous conditions. The fiber characteristics of the muscles which have been studied in the heterozygous animals are usually intermediate but closer to those of the normal animal. Since differences in muscle fiber characteristics do not appear to be a plausible basis for separation of the normal and heterozygous DM genotypes, other techniques will be needed. Swatland has recently reported that DM animals have a higher proportion of branched terminals axons than do normal animals. It is possible that this and other parameters, in combination with muscle fiber characteristics, could be used for genotypic classification of animals relative to the heterozygous DM condition.