The effects of cow’s genotype at αS1- and αS2-casein gene 5’-noncoding regions (promoters) were determined on selected milk production traits of the 135 Polish Black-and-White (Polish Friesian) cows as related to the animal’s age, lactation parity and stage, and somatic cell count. Cows of the AA genotype of αS1-casein gene yielded more milk daily than AG heterozygotes. Also, the daily yield of solids-non-fat, protein and lactose was higher in AA genotype cows. Milk of the cows with genotype CC of αS2-casein contained more lactose but less protein than that of the CT heterozygotes. The daily protein yield was slightly (but significantly) higher in the cows of the CT αS2-casein genotype. In summary, the results showed that genetic variants of αS1- and αS2-casein 5’-noncoding regions had only a slight effect on milk production traits of the Polish Black-and-White cows. Nevertheless,the AA genotype of αS1-casein seemed favourable for higher milk yield, as well as for lactose and protein content.
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