The article is devoted to one of the important problems of improving the safety and vitality of young animals. Progress in biotechnology and molecular genetics allows one to give an objective assessment of the genetic potential, to predict the resistance of the animal organism to diseases, unfavorable environmental factors at the early stages of postnatal ontogenesis. One of the advantages of DNA diagnostics is the ability to assess animals at birth. DNA diagnostics allows to obtain a large amount of information in a short time, as well as to determine the polymorphism of genes that control the characteristics of productivity in sheep. This allows you to objectively assess the prospects of young animals and select animals for inclusion in the selection process, in the early period of ontogenesis. In this regard, the goal was set to study the polymorphism of the somatotropin - GH, calpastatin - CAST genes and its relationship with the parameters of the immune reactivity of lambs of different genotypes. Using the PCR-RFLP method, polymorphism of genes, GH, CAST, represented by two GHA alleles, was revealed; GHB and CASTM; СASTN with different frequency of occurrence and three genotypes: GHAA, GHBB, GHAB; СASTММ, СASTNN, СASTМN. Taking into account the results of genotyping, experimental groups were formed: I - carriers of GHAA; II - GHBB; III - GHAB; IV - СASTММ; V– СASTNN; VI - СASTМN genotype. The genotypes of the carriers of marker alleles were revealed. Immune reactivity was assessed by the level of immunocompetent cells (T-, B-lymphocytes) and the ratio of their subpopulations (T-helpers; T-suppressors). The relationship between the size of live weight, average daily gains, and immune reactivity of different genotypes was established. The superiority in terms of live weight of lambs with genotypes GHBB and GHAB compared to GHAA CASTMN; which averaged 0.9 and 5.8%. The revealed regularity can serve as an evaluative criterion of the viability and productivity of sheep at an early age.

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The presence of coarse fiber in newborn animals indicates the viability and high productivity of animals in adulthood, therefore, when breeding meat-wool sheep of the Jalghin merino breed, flocks of lambs should be formed according to the amount of coarse fiber at birth. It was found that in all groups, animals born with coarse fiber at all age periods had superiority in live weight over animals born without coarse fiber. In group I of the fine line, 72.5% were born without coarse fiber, and 27.5% with coarse fiber. In group III of the strong line, 13.6% were born without coarse fiber of lambs, 86.4% with coarse fiber. In the second group of the medium line, there was practically the same ratio of without and with coarse fiber lambs and amounted to 48.7% and 51.3%, respectively. It was found that, at birth and at 4 months of age, in terms of live weight, rams with coarse fiber, regardless of their linearity, outnumbered their peers. At 14 months of age, the superiority of animals of group III in live weight in general over their peers in groups I and II was 8.8% (P <0.001) and 5.4% (P <0.001). In turn, the rams of the III group that were born with coarse fiber had a significant superiority over the coarse-fibered peers of the I and II groups by 5.6% (P <0.001) and 2.8% (P <0.001). The same tendency was noted for the rams that were born without coarse fiber, animals of the III group (strong) had a significant superiority over their peers (fine and medium).

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