STUDY OF BETA-CASEIN GENE POLYMORPHISM AND ITS RELATIONSHIP WITH MILK COMPOSITION IN SIMMENTAL COWS
In these days, scientists increasingly frequently focus their attention not only on the quantitative indicators of milk productivity of cows, but mainly on the qualitative ones. It became a matter of topical interest after the negative effect of milk on human health was discovered. In recent years, researchers have noted that the consumption of cow's milk can cause disorders of the gastrointestinal tract which can be due to the presence of β-casein A1 in milk. There are more than 5 variants of β-casein in milk. However, two of them, A1 and A2, are the most common. In cows of different breeds, the frequency of alleles differs significantly. In animals of the world's most widespread Holstein breed, the frequency of the desired allele is in the range of 0.5–0.6. The frequency of the desired genotype is in the range of 35–40%. In animals of the Swiss breed, the frequency of the desired A2 allele is significantly higher – 0.70–0.75, and the desired genotype A2A2 is about 70%. In African and Asian cattle breeds, the frequency of the A2 allele and the A2A2 genotype is very high and is in the range of 0.90–0.98. Today some countries of the world create dairy herds where homozygous animals A2A2 are bred . At the same time, consumers are interested in such products despite the fact that they are 1.5–2 times more expensive.
The aim of the research was to study the polymorphism of the beta-casein gene in Simmental cows and its relationship with the quality indicators of their milk productivity.
Materials and methods. Genotyping of Simmental cows bred on the stud farm "Mykhailivka" in Lebedyn district of Sumy region (n = 46) and private agricultural enterprise "Urozhai" in Romny district of Sumy region (n = 30) was carried out. Determination of beta-casein gene polymorphism was performed in the genetic laboratory of Bohomolets Institute of Physiology of NAS using molecular biological analysis of allele recognition by polymerase chain reaction (PCR) in real time.
To collect milk samples a counter indicator IU-1 was used. The milk sample was stored in a plastic container (25 ml). The biochemical composition of milk from seven cows of each genotype (A1A1, A1A2, A2A2) was analysed. The fat and protein content in milk was determined in the laboratory of Sumy National Agrarian University on the equipment Ultrasonic milk analyser Master
The results of the research were processed by the methods of mathematical statistics by means of the package "Statistica-6.1" in the Windows environment on a PC.
Analysis of genotyping data of Simmental animals revealed that a higher frequency is characteristic of genotypes A1A2 and A2A2 being more than 40%. As a result, the frequency of the desired A2 allele was quite high. The calculation according to the Hardy-Weinberg formula showed no difference between the actual and expected frequencies of genotypes in animals. Using genetic and statistical methods of analysis, by determining the numerical values of such genetic constants as the degree of homozygosity (Ca), the level of polymorphism (Na), we tried to assess the prospects of increasing the frequency of the desired A2A2 genotype in the Simmental population. The degree of homozygosity in the studied populations of cattle is 55.3%, which may indicate a sufficient level of their consolidation. This is confirmed by the analysis of such genetic indicators as the level of polymorphism (the number of effective active alleles – Na), which was equal to 1.84 with a maximum of 2. The heterozygosity test (TG), which shows the level of genetic diversity of the population, in the studied herds was almost zero, which indicates a balance between the share of actual heterozygotes relative to the share of theoretical heterozygotes. As for the coefficient of excess (D), which characterizes the ratio of actual heterozygosity to theoretical, we note a slight deviation of the actual heterozygosity from the expected with left-hand excess, which also indicates the absence of a deficit of heterozygotes.
The average content of milk constituents in the cows of all studied genotypes corresponded to the breed standard. Animals of the Simmental breed with the desired A2A2 genotype slightly prevailed animals of other genotypes in all studied indicators, but the difference between them was statistically insignificant. Therefore, in our opinion, the creation of herds with the A2A2 genotype for β-casein will not have a negative effect on the productive traits of cows and thus ensure the preservation of high quality milk indicators of new type cattle herds.
Conclusions. Genotyping of Simmental breed by β-casein gene was conducted. It was found that the A2 allele (0.645) was characterized by a higher frequency. Respectively, the frequencies of genotypes A1A1, A1A2 and A2A2 were 13%, 45%, 42%. According to genetic and statistical analysis, the balance between the actual and theoretical distribution of genotypes has been determined.
In terms of milk fat and protein content, animals of all genotypes met the breed standard. No statistically significant difference in the quality characteristics of milk was found between animals of different genotypes. It can be stated that the formation of herds with the A2A2 genotype for beta-casein will not have a negative impact on the productive characteristics of cows and thus ensure the preservation of high quality milk of new herds. However, the study of the influence of the beta-casein genotype on the technological properties of milk requires further study.
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