This study aimed to investigate the effect of A and B alleles of beta-lactoglobulin (BLG) gene on milk-related traits in small ruminants by using data reported in 26 potential articles. Records from 9439 genotyped sheep and goats were analysed by utilising four genetic models, including dominant (AA þ AB vs BB), recessive (AA vs AB þ BB), additive (AA vs BB), and co-dominant (AAþ BB vs AB). The effect size of each genotype on daily milk yield, fat percentage, and protein percentage was measured as a standardised mean difference (SMD). The fixed-effects or random-effects model was used to analyse data based on the results of the heterogeneity test. The results showed that B allele of BLG gene positively, but slightly increased the fat percentage of milk in combined data of sheep and goat under dominant (SMD ¼ 0.207, p ¼ .018), recessive (SMD ¼ 0.190, p ¼ .000) and additive (SMD ¼ 0.699, p ¼ .003) genetic models. Also, a statistically significant effect of B allele on daily milk yield of goat was found under dominant (SMD ¼ 0.208, p ¼ .001) and co-dominant (SMD ¼ 0.149, p ¼ .002) models. There was no association between studied alleles and milk protein percentage in sheep and goat. Results of publication bias tests showed a lack of bias and sensitivity analyses suggested additive model as the best one and animals with BB genotype showed highest performance in milk fat production with a SMD equal to 0.699.
