In assessing single gene effects on a quantitative trait, contrasts between the genotypes would be of primary interest. However, only the net differences over and above the contributions from polygene effects from other loci would be of practical importance. In a population of 11015 Holstein cows, the B allele ofK-casein was significantly favourable for milk and protein yields and for protein %, and the A allele of β-lactoglobulin was favourable for fat % and protein yield. The degree of dominance for their effects was shown. Also shown were the independence between effects of the casein variants and those of polygene, and between effects of casein genes. Adjustment for casein gene effects made no difference to the estimates of heritability and other genetic parameters of all five dairy production traits.