Genetic polymorphisms of kappa casein gene and its association with milk and composition traits in cows: An updated meta-analysis

Abstract

The present meta-analysis was planned to summarize the frequency and effects of A and B alleles of kappa casein gene on milk yield and composition traits by using information of 5715 genotyped cows from 42 published studies (2000 to 2020). The common effect sizes for gene frequency and association were considered as proportion and standardized mean differences (SMDs). Four genetic models viz., additive (AA vs. BB), dominant (AA+ AB vs. BB), completely over dominant (AA+BB vs. AB) and recessive (AA vs. AB+ BB) were used to compare the potential of genotypes in terms of SMDs along with 95% confidence interval (CI) for lactation milk yield and composition traits (fat yield, fat percentage, protein yield and protein percentage). The inconsistency between studies was estimated by heterogeneity statistic (I²). Meta-analysis of allelic frequency under random effects model showed that allele A was predominant as 0.71 (95% CI: 0.65, 0.76) in all genotyped cows. It was significantly higher gene frequency in 1834 Bos indicus cows (0.82, 95% CI: 0.77, 0.88) as compared to 3881 Bos taurus/cross cows (0.67, 95% CI: 0.61, 0.73), with substantial level of heterogeneity (92.73% to 97.68%). The results of association analysis showed that SMDs under all genetic models had significance (P < 0.05) with fat percentage only, with non-significant of heterogeneity (P > 0.05) between studies. For other composition traits and lactation milk yield, non-significant SMDs were observed with low to moderate heterogeneity index. There was not any risk of publication bias as confirmed from Egger's test (P > 0.05). It was concluded that significant molecular marker (allele B) of kappa casein can be used to improve milk fat percentage in dairy cows.