Meta-analysis of SNP in growth hormone gene associated with milk traits in dairy cows

Previous studies reported varied findings regarding growth hormone polymorphisms and their association with milk traits in dairy cattle. The objective of the present study was to combine the information of Alu1-derived gene polymorphisms of growth hormone in 6571 dairy cows using 51 published studies for obtaining pooled estimate of gene frequency of L allele in dairy cows and for deriving common effect size of association of gene variants with various milk traits (lactation milk yield, fat yield, fat%, protein yield, and protein%) using meta-analysis. The pooled estimate of gene frequency was obtained using proportion as effect size with 95% confidence interval under random effects model. For association analysis, standardized mean difference was employed as effect size under four genetic models comprising additive (LL vs. VV), dominant (LL + LV vs. VV), completely over dominant (LL + VV vs. LV), and recessive (LL vs. LV + VV). The heterogeneity between studies was estimated using Q test and I² statistic. The results showed that pooled estimate of gene frequency of L allele was 0.81 (95% CI: 0.77, 0.85; I² = 96.86%). Subgroup analysis revealed that it was significantly higher in Bos indicus cows (0.97, 95% CI 0.89, 1.00; I² = 95.15%) than Bos taurus/cross cows (0.78, 95% CI 0.73, 0.82; I² = 96.84%). An association analysis showed that the effects of GH variants on milk-related traits were not significant (P > 0.1) under all genetic models. Substantial range of heterogeneity indicated modest effects of SNP genotypes with milk traits. It was concluded that although allele L is predominant than V allele of GH in dairy cows over large geographical populations, they may not be particularly useful for selection programs for improving milk traits in dairy cows.