Analysis of β-casein gene for A1 and A2 allelic variants in Holstein-Local crossbred cattle of Bangladesh using allele-specific PCR

β-casein (CSN2) is one of the important bovine milk proteins synthesized by β-casein gene. The aim of this study was to investigate the A1 and A2 allelic variants of CSN2 gene in Holstein-Friesian and Local (HF×L) crossbred cattle of Bangladesh. Allele specific polymerase chain reaction (AS-PCR) was carried out for genotyping of allelic variants. A total of 390 blood samples of HF×L crossbred cattle was included from government, military, and private dairy farms. Six different crossbred genotypes were identified in the studied population such as 50%HF×50%L, 62.5%HF×37.5%L, 68.75%HF×31.25%L, 75%HF×25%L, 81.25%HF×18.75%L, and 87.5%HF×12.5%L. The mean frequencies of the A1A1, A1A2, and A2A2 genotypes were 0.11, 0.48, and 0.41, respectively, while the corresponding frequencies of the A1 and A2 alleles were 0.34 and 0.66, respectively. The frequency of desirable A2A2 genotype reduced from 0.60 (50%HF×50%L) to 0.38 (87.5%HF×12.5%L) with the advancement of exotic HF inheritance from 50 to 87.5%. In contrast, the frequency of A1A1 and A1A2 genotypes increased proportionately with the progression of HF inheritance level. The undesirable A1A1 genotype frequency was found highest in the HF crossbreds of private dairy farms (0.13) compared to government (0.08) and military dairy farm (0.07) that signifies the availability of upper graded cows in the private dairy farms. Altogether, this study provides current scenario of A1 and A2 allelic variants of HF×L crossbreds of Bangladesh that could be utilized to design suitable breeding plans in crossbreeding programs for production of premium quality milk.