Polymorphism of kappa casein gene (κ-Cn) at exon 5 in 259 Chinese Holstein cattle was investigated using polymerase chain reaction-single strand conformation (PCR-SSCP) technique, to verify its effect on milk composition (fat, protein, lactose, total solids (TS) and solids not fat (SNF) content and influence of other factors such as parity and season of calving; aiming at utilizing it as a genetic aid in selection to improve the quality of production of this herd. A 218 bp fragment containing exon 1V of kappa casein gene was amplified by PCR and (SSCP) was applied to identify the structural gene polymorphism of κ-Cn. General linear model (GLM) was used to analyze differences between (κ-Cn) genotypes. The results indicated that κ-Cn genotypes significantly (P<0.05) affected fat, protein and lactose content. But it had not influenced (P>0.05) both TS and SNF. However, cows with genotype TT showed higher fat and protein percent (3.89 and 3.50%, respectively) than those of genotypes CC and TC (3.62 and 3.66% for fat; 3.35 and 3.36 % for protein, respectively). Except for lactose content; parity had no effect on milk components. Also, the results disclosed that season of calving did not affect (P>0.05) milk composition. This study indicated that the κ-CN genetic variants may be used as a genetic aid through increasing the frequency of desired genotypes to improve the quality of production of this herd.

Key words: Kappa casein genetic variance; Milk total solids and non solids; Parity; Season of calving