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Single nucleotide polymorphism of the β-lactoglobulin gene in sheep breeds reared in Bulgaria

In the present study, single nucleotide polymorphism in exon II of the β-lactoglobulin gene was investigated in four Bulgarian sheep breeds: Bulgarian Dairy Synthetic Population sheep (BDSP), Copper-red Shumen sheep (CRSH), Stara Zagora sheep (STZG) and Pleven Blackhead sheep (PLBH). Two genetic variants of β-LG gene (A and B) have been identified through PCR-RFLP assay. A 103 bp fragment of the polymorphic β-LG locus were amplified and digested with endonuclease enzyme RsaI. The obtained restriction fragments revealed three genotypes: AA, AB and BB, observed in 31%, 65.5% and 3.5% of the BDSP population and in 48%, 28% and 24% of CRSH sheep population, respectively, with departure from the Hardy-Weinberg equilibrium (P<0.05) in these groups. The allele frequencies demonstrated a prevalence of the A allele (0.638 and 0.620) over the B allele (0.362 and 0.380) in both populations. On the contrary, the distribution of allele frequencies in STZG and PLBH was 0.240 and 0.100 for allele A, respectively and 0.760 and 0.900 for allele B. Therefore, the homozygous BB genotype in these sheep populations was more frequently encountered (0.520 and 0.800) than the heterozygous AB genotype (0.480 and 0.200), with HWE correspondence (P>0.1). The homozygous genotype AA was absent in STZG and PLBH sheep populations. The greatest Nei’s genetic distance calculated by UPGMA method was found between the populations BDSPand PLBH (0.5334), while the closest relationship (0.0006) was established between CRSH and BDSP. The results obtained from the present investigation confirmed the presence of the SNP polymorphism in exon II of the β-lactoglobulin gene. Therefore, the genetic variability established in this polymorphic locus could be applied in further association studies with milk production traits in sheep.