Abstract
Most of the economically important traits in dairy cattle are quantitative in nature, which means that they are affected by environmental factors and by large number of gene. Selection of superior animals has been made more effective through studies of major milk protein genes that are known to affect both milk yield and composition. The present study was carried out to detect polymorphism in kappa casein (CSN3) and beta-lactoglobulin (LGB) genes in Holstein cattle under Egyptian condition through DNA sequencing and polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP). Fifty Animals were divided into high and low milk producing according to their breeding value. PCR amplification of exon III o CSN3 and LGB was performed followed by restriction fragment length polymorphism (PCR-RFLP) using Hind-III and HinfI for CSN3 and HaeIII restriction endonuclease for LGB. Nucleotide polymorphisms between high and low producing cows were detected by DNA sequencing. The restriction enzymes digestion failed to produce restriction patterns and revealed no polymorphism in all studied animals. Comparison of nucleotide sequences between high and low producing cows revealed lack of polymorphism in CSN3 and four nucleotide changes in LGB gene; C179 T, C225T, T246C, and C294G. The further study using other specific restriction endonuclease was required to detect polymorphisms of CSN3 and LGB in Egyptian Holstein cattle. SNPs discovered in this study can be used as molecular genetic markers for marker assisted selection (MAS) to increase and accelerate the rate of genetic improvement of milk production traits.

Key words: kappa-casein, beta-lactoglobulin, Holstein cattle, PCR-RFLP.