Recessive Effect of GC-NPFFR2 rs137147462 on Somatic Cell Score (Mastitis Susceptibility) in Japanese Holsteins

We evaluated four candidate SNPs (<i>GC-NPFFR2</i> rs137147462, <i>GC-NPFFR2</i> rs109452259, <i>BRCA1</i> rs134817801, and <i>DGAT1</i> p.K232A) previously reported in relation to mastitis or milk production traits, using 10,729 test-day phenotypic records collected over 10 years from 269 Japanese Holstein cows (<i>Bos taurus</i>) enrolled in the national Dairy Herd Improvement (DHI) program. Linear mixed models were used to estimate genotypic effects on somatic cell score (SCS) and to test multiple inheritance models. To assess clinical relevance, mastitis severity was further analyzed using categories defined by somatic cell counts (SCC). Among the SNPs tested, <i>GC-NPFFR2</i> rs137147462 showed the clearest and most consistent association with SCS under a recessive model, with GG cows exhibiting higher SCS throughout lactation. Ordinal logistic regression confirmed a higher probability of progression to severe mastitis in GG cows. <i>DGAT1</i> p.K232A showed additive effects, with the A allele increasing milk yield while lowering fat and protein percentages. AA cows also showed higher SCS under a modest recessive effect. <i>BRCA1</i> rs134817801 and <i>GC-NPFFR2</i> rs109452259 had minimal effects. These findings support <i>GC-NPFFR2</i> rs137147462 as a promising marker for mastitis resistance and indicate the importance of considering not only additive but also recessive genetic models in genomic selection strategies.