Canadian scientists recently reported a polymorphism in the leptin gene of cattle that is associated with a change in body composition, most notably an increase in marbling. In this South Dakota State Univ. study, genotypes for the polymorphism were determined on 492 crossbred calves. Individual animals were genotyped as CC (homozygous for the polymorphism), CR (heterozygous), or RR. Of the 492 calves, 118, 276, and 98 were CC, CR, and RR, respectively. After weaning, the calves were finished in a commercial feedyard. Carcass traits were evaluated at a packing facility at an average age of 433 days. Carcass data were adjusted to three different endpoints: carcass wt. (723 lb), fat thickness (.45 in.), and age (433 days) in three separate analyses.

Genotype was not significantly associated with carcass wt. or ribeye area in any of the three analyses. Associations of genotype with fat thickness, KPH fat, and cutability were small and generally not statistically significant. However, associations of genotype with marbling score were statistically significant in the age- and fat-adjusted analyses and approached significance in the weight-adjusted analysis. Individuals with two copies of the C allele had marbling scores of about 20 units higher than those with zero copies (approximately 407 vs. 386). Those with one copy were somewhere in between. The authors concluded that the magnitude of difference observed in this study may be sufficient to justify genotyping of herds or populations in which most market animals have marbling scores near a quality grade/price threshold (Bierman et al. 2003. South Dakota Beef Report).