Abstract：In order to study DRB1 gene exon 3 polymorphism in Tibetan sheep, The number of alleles, single nucleotide polymorphisms(SNPs) sites, variation type, the genetic relationship and evolutionary significance of the alleles had been analyzed. Variation in the 500 Tibetan sheep (Ovis aries) DRB1 gene exon 3 was investigated by PCR-SSCP method, followed by cloning and DNA sequencing. The results showed that 8 alleles were identified, 15 nucleotide polymorphism sites were identified in 8 sheep DRB1 gene haplotypes and 7 novel DRB1 exon 3 alleles were found comparison with GenBank sequences. The phylogenetic tree results showed that the 8 haplotypes of Tibetan sheep DRB1 exon 3 could divide into two clusters. Allele B was the most common allele in three Tibetan sheep populations, and that allelic and genotype frequency were significantly deviated from Hardy-Weinberg balanced. The polymorphism was higher with PIC more than 0.5 at DRB1 gene exon 8 in three Tibetan sheep populations. The results suggest that Tibetan sheep DRB1 gene exon 3 has high level of sequence polymorphism，Tibetan sheep DRB1 gene is differentiated into two major category alleles from two mutant alleles at first.
