| Κappa-casein protein is phosphoprotein containing a small amount of phosphate that secreted by a cow mammary gland normally. It is a composition of casein in the milk and natural substrate of chymosin. In the raw state,κ-CN is an important factor to maintain a stable milk emulsion status.Theκ-casein gene was studied in 600 China Holsteins and regarded as a candidate gene for milk performance traits of cows. Sequencing,PCR-RFLP and CRS-PCR were applied to analyze the relationship between polymorphisms of 12 loci inκ–casein gene exon 4,5 and intron 4 and haplotype combinations of exon 4 and 5 in Chinese Holstein cattles and the milk performance traits for the purpose of providing molecular maker information to facilitate the breeding efficiency of high milk protein and milk fat. The results were as follows:1. Genetic polymorphisms ofκ–casein gene exon 4 and its correlation with milk performance traits in Chinese HolsteinsIn this study, the polymorphisms of four loci ofκ–casein gene exon4 were detected by PCR-RFLP and CRS-PCR with restriction endonuclease TaqI,HindШ,PstI,SspI. After sequencing, T/C,C/A,G/A and T/A SNP were identified at nucleotide 10891,10927,10988,10996 in exon4 ofκ–casein gene. The polymorphisms of the four loci in the experimental population is closely linked. Both alleles (A and B) of four loci were found in the population that showed low polymorphism. The gene frequencies of A and B were 86.03% and 13.97% separately. The genotype frequencies of AA, AB, BB were 73.71%, 24.63% and 1.66% respectively. Statistical results ofχ2 test indicated that four polymorphism sites in the population fitted with Hardy–Weinberg equilibrium (P > 0.05). Meanwhile, the effect of polymorphism ofκ–casein gene on milk production traits was analyzed, the results indicated that in the four loci, the cows with genotype BB and AB showed higher milk fat percent than those with genotypes AA(P < 0.05) ; with genotype AB showed higher fat protein ratio than those with genotypes AA(P < 0.05) ; the different genotypes ofκ–casein gene had no significant influence on milk yield and milk protein percent (P > 0.05).2. Genetic polymorphisms ofκ–casein gene exon 5 and haplotype combinations of exon 4 and 5 and its correlation with milk performance traits in Chinese HolsteinsFive SNPs in the exon five(A12907G,G12950A,C12989T,A13028G,T12980C) were found and A12907G,G12950A,C12989T,A13028G were closely linked. At locus T12980C, the cows with genotype TC and CC was very significantly (P < 0.01)and significantly higher than TT genotype, respectively (P < 0.05) ; with genotype TC and CC showed higher protein rate than those with genotypes TT(P < 0.05). At the remaining four loci, the cows with genotype DE showed higher fat rate than those with genotypes DD(P < 0.01). Haplotype combinations in theκ–casein gene exon four and five were studied. 7 haplotype and 16 haplotype combinations were found in 398 Chinese Holstein cattles, the cows with haplotype combinations H6H6 had the highest milk fat percentage and secondly was haplotype combinations H1H4. The cows with haplotype combinations H1H4 had the highest milk protein percentage and with haplotype combinations H5H6 had the lowest milk fat percentage and milk protein percentage.3. Genetic polymorphisms ofκ–casein gene intron4 and its correlation with milk performance traits in Chinese HolsteinsThree SNPs in the intron four(A12817G,G12218A,A12016G) were found by sequencing. The effect of polymorphism ofκ–casein gene on milk production traits was analyzed, the results indicated that in the A12817G loci, the cows with genotype AG showed higher fat rate than those with genotypes AA(P < 0.01); in the G12218A loci, the cows with genotype GA showed higher milk fat percent and protein rate than those with genotypes AA(P < 0.05); in the A12016G loci,the different genotype ofκ–casein gene had no significant influence on milk production traits (P > 0.05).
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