Abstract
The results of the use of genetic markers of the first and second types in the breeding of farm animals and veterinary medicine are reviewed. The results of the study of relationships between mendelating candidate genes and markers with alleles of complex traits are contradictory. To increase the effectiveness of research, a comprehensive population-hybridological approach to the analysis of relationships of the first type markers with breeding traits is proposed. The effectiveness of this approach is demonstrated by studying the relationship between milk protein polymorphism and the signs of milk productivity in the population of Kholmogorsky cattle. According to β-Cn (CSN2), the association of the β-CnA2 allele with increased milk productivity of cows was established at the population level. The genetic determination of the identified association was confirmed by family (segregation) and twin analysis. No associations were found for β-Lg (LGB) at the level of the population as a whole and for subpopulations stratified by breeding status. The relationships of β-lactoglobulin polymorphism with milk yield in the samples representing individual breeding plants turned out to be multidirectional, but statistically significant in five cases out of five at a significance level of P<0.05. In the herds of the two leading breeding plants, the "marker effects" remained unchanged by sign for several generations. With perfect similarity in the frequencies of the β-Lg genes, opposite associations were observed in these breeding plant herds. In the offspring of heterozygous producers from the analyzing crosses between the mendelating genes and the quantitative trait polygenes, the associations characteristic of the populations from which the heterozygotes originated were preserved in the direction. The level of selection differentiation of marker alleles depended on the level of average milk productivity of the herd. The paper discusses the influence of environmental and genetic factors on the formation of associations. It is concluded that it is impossible to focus research on obtaining "average" effects of genes in "average" conditions, abstracted from specific genotypic and environmental situations of their manifestation. The solution to this problem is to improve the research ideology, which is currently limited to the registration of reliable associations of markers with polygenic quantitative traits in retrospective reference samples, followed by extrapolation of the results obtained to genetically different populations located in other environmental conditions. In animal breeding (for example, with plant selection), it is necessary to develop a theory of the ecological and genetic organization of a quantitative trait, since the structure and relative contribution of genes to the manifestation of a trait can change depending on variations in environmental conditions.
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