IntroductionGenetic research in farm animals focuses mainly on the identification of genes influencing economically important traits that could be useful in breeding programs. In dairy cattle, most studies have researched the genes that may determine variations in milk yield and milk composition (Komisarek and Dorynek, 2009). The polymorphisms of several genes including growth hormone (GH), prolactin (PRL), and diacylglycerol acyltransferase (DGAT1) have been shown to affect milk yield and milk composition traits in dairy cattle.GH belongs to a family of somatolactogenic hormones including prolactin, placental lactogen, and a number of hematopoietic growth factors (Cosman et al., 1990). GH is located on the 19th chromosome in the q26-qter band region (Hediger et al., 1990). The GH gene of cattle is approximately 1800 bp in size and contains 5 exons and 4 introns. This gene encodes an mRNA with a size of 786 bp (Woychik et al., 1982). Although a number of polymorphisms have been observed in the GH gene of cattle, the 2 polymorphisms that are located in intron 3 and exon 5 have been found to be significant in their effects on milk and meat yield parameters (Lucy et al., 1991(Lucy et al., , 1993Høj et al., 1993). The polymorphism in intron 3 of GH is observed when GH is digested by the restriction enzyme MspI (GH-MspI) (Zhang et al., 1993). Two alleles are observed as a result of digestion with this enzyme. The first allele, GH-MspI ( -) contains a T insertion at the +837 position, and the second allele has a C-G transition at the +837 position (Lee et al., 1994). Zhang et al. (1993b) reported that the polymorphism in exon 5 could be observed when digested by the AluI enzyme (GH-AluI) and the 2 alleles called L (leucine in the 127th codon) and V (valine in the 127th codon) occur.The PRL gene is located on chromosome 23 and is composed of 5 exons and 4 introns (Camper et al., 1984, as cited by Skinkyte et al., 2005. A silent A-G mutation of the 103rd codon in exon 3 of the bovine PRL gene led to a polymorphic RsaI site (Lewin, 1992, as cited by Skinkyte et al., 2005). The polymorphism leading to the RsaI enzyme restriction site of the bovine PRL gene determines milk production traits including milk yield, milk fat, and milk protein contents (Lewin et al., 1992). The significant effect of the PRL locus on milk production traits was later shown by Chung et al. (1996, as cited by Dybus, 2002a as well.
