Vol. 8, Issue 5 (2019)

Author(s):
Baddula Srinivas, Kumar K Aswani, K Padmaja and B Punyakumari

Abstract:
The exotic and cross bred cattle milk contain A1 β-casein variant, whereas indigenous cattle milk contains A2 β-casein variant. They have structural diversity at 67th amino acid residue proline in A2 which is replaced by histidine in A1 during course of evolution. The histidine at 67th amino acid residue generates a proteolytic cleavage site in A1 leading to the production of betacasomorphin-7 (BCM-7) that is known to be a risk factor for several human diseases. The serious health concern associated with A1 milk consumption endures an urgent need to select cattle breeds with higher genotypic and allelic frequencies of A2 and conserve indigenous cattle breeds through breeding programmes. In this context, the present study was undertaken to screen thirtysix animals belonging to two indigenous cattle breeds of India viz., Ongole, Punganur and Holstein Friesan cross (twelve from each breed) to characterize β-casein variants (A1 and A2) geotypically by PCR-RFLP with TaqI restriction enzyme and also to study milk β-casein variants (A1 and A2) by IEF. PCR-RFLP with Taq1 restriction enzyme yielded two genotypes as A1A2 (251 and 213 bp) and A2A2 (251 bp) in all indigenous cattle breeds studied, whereas only heterozygous (A1A2) genotype was found in the HF cross. The genotypic frequency of A1A2 heterozygous variant of exon 7 of CSN2 gene was the highest for HF cross (1.0) followed by Punganur (0.17) and Ongole (0.083). Whereas, for A2A2 homozygous variant was the highest for Ongole (0.92) followed by Punganur (0.83). The A1 allelic frequency of exon7 of CSN2 gene was the highest in HF cross (0.50) followed by Punganur (0.083) and Ongole (0.042), whereas the highest A2 allelic frequency was observed in Ongole (0.96) followed by Punganur (0.92) and HF cross (0.50). The IEF gel electrophoresis pattern of milk β-casein protein showed two distinct bands corresponding to A1 and A2 β-casein protein variants at pI values 5.22 and 5.14 respectively and this polymorphism correlated positively with the results of PCR-RFLP with TaqI restriction enzyme. The present study unveils the existing fact that there is incorporation of exotic germplasm in native cattle breeds to varying degrees in different breeds and emphasizes an immediate need for genotypic screening of bulls for β-casein gene variant A2 for documentation and further measures to improve the indigenous breeds with appropriate breeding strategies.