The undertaken study aimed at determining relations between kappa-casein polymorphism and milk performance traits in population of 304 Holstein-Friesian heifer cows bred in fi ve herds in the Kuyavian-Pomeranian region. Numerical data were fi rst verifi ed statistically with the GLM procedure, incorporating the effect of basic factors (herd, date of birth, calving season, share of hf breed in the cows genotype and genotype of kappa-casein gene) on milk performance
traits. The effect of some factors on traits was highly signifi cant. Genotype of CASK gene was highly signifi cant on almost all verifi ed milk performance traits. Based on the kappa-casein gene polymorphism, the investigated population was divided into three groups: AA, AB, BB with the share in population of 216, 70 and 18, respectively. The allele A and B frequency was 0,83 and 0,17 respectively. Cows with the AA genotype of kappa-casein were characterized by the highest milk (6414kg), fat (217kg) and protein (209kg) yield, while the lowest fat and protein contents were
observed in milk of cows with the BB genotype.
