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Milk samples from 1454 first lactation heifers were collected from five breeds of cattle in a representative sampling of the California dairy cattle population. Samples were phenotyped for α_s1-casein, β-casein, κ-casein, and β-lactoglobulin using PAGE. Lactation yields for milk, fat, and protein were available on 1166 heifers. Genetic linkage of the casein loci and the relationship between milk protein genotypes and production parameters (milk, fat and protein yield, fat and protein percentage) was investigated. Gene frequencies were similar to those found in other studies of the American dairy cattle population. Linkage was found between β-casein and κ-casein in four of the five breeds and between β-casein and α_s1-casein in the Jersey breed. Casein haplotypes BBB and CA³A (α_s1-, β-, κ-casein respectively) occurred in the Holstein population more often than would be expected from a random combination of the …