An MspI polymorphism at the bovine growth hormone (bGH) gene is linked to a locus affecting milk protein percentage.

SSCP analysis of the bovine growth hormone (bGH) gene in Israel Holstein dairy cattle uncovered five intragenic haplotypes, denoted A to E. Of these, Haplotype E differed from the others at six fragments; one of which corresponded to the polymorphic MspI site in intron III, at which haplotype E carried the disabled MspI (-) allele. Haplotype E was observed in a single sire only, carrying haplotype A as the second bGH allele. In 523 daughters of this sire genotyped for the MspI polymorphism, heterozygous (+/-) as compared to homozygous (+/+) daughters, showed a significant increasing effect on protein percentage and kg protein per year; and a decreasing effect (P < 0.10) on milk somatic cell counts (MSSC). None of the daughters were homozygous (-/-), indicating that the frequency of this allele in the general population was essentially zero. Calculated skewness (g1) values for the two daughter groups differed significantly with (+/-) daughters showing negative skewness (in the direction of lower protein percentage), and (+/+) daughters positive skewness (in the direction of higher protein percentage). The direction of skewness in each group is indicative of the presence of a QTL having an increasing effect on milk protein percentage in coupling linkage with the MspI (-) allele in this sire, but at some distance from it. Maximum likelihood estimates of the proportion of recombination (r) between the putative QTL and bGH, and the allele substitution effect at the QTL (d), were r = 0.33, a = 0.07% protein, with standard errors 0.058 and 0.009% protein, respectively.