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The research presents the findings of DNA testing of allelic polymorphism by the PCR-RFLP method of the SCD1 gene and the relationship of genotypes with the milk fatty acid composition of Holstein cows of Tatarstan population. The research was conducted among 172 Holstein cows at the Integrated Agricultural Production Centre “Stud farm named after Lenin” in Atninsky district of the Republic of Tatarstan in 2017–2018. All genotypes of the SCD1 gene were identified through the research. The milk yield test showed that the maximum content of the mass fraction of fat was in animals with the TT genotype. The significant difference between the content of the mass fraction of fat in milk in individuals by the locus of the SCD1-Fsp4H I gene was: TT to CC – 0.32 % (p < 0.05) and TT to TC – 0.40 % (p < 0.01). The results of chromatography indicate the balance of the milk fatty acid composition of cows with the TT genotype and, as a consequence, its positive effect on the human body. Cows with a homozygous TT genotype are characterized by the optimum ratio of fatty acids in milk fat. The maximum value of the oxygen desaturation index is also observed in animals with the TT genotype by the SCD1 gene.

The study presents the results obtained in DNA testing of allelic polymorphism by PCR-RFLP based methods for the leptin and stearoyl-coenzyme A desaturase genes and the interaction of their complex genotypes on the milk productivity indicators of Holstein heifers of Tatarstan population. The study was carried out among 471 Holstein cow-heifers by Integrated Agricultural Production Centre “Stud farm named after Lenin” of Atninsky district of the Republic of Tatarstan in 2016–2018. All complex LEP/SCD1 genotypes were identified through the study. The maximum frequency was observed in the combination of heterozygous genotypes LEPTCSCD1TC 149 animals (31.5 %). Milk productivity test, based on the complex genotypes LEP and SCD1, showed that a significantly high level of productivity for 305 days of lactation was 7314.4 kg, and in heifers with a complex genotype LEPTTSCD1CC was minimal (6337.4 kg) in comparison with indicators of milk yield of heifers with other complex genotypes. According to the content of the mass fraction of fat, reliably the best indicators were established in a subpopulation of animals with a complex TT/TC genotype 3.87 %. The content of the mass fraction of protein was maximal for animals with a complex CC/CC genotype.