In cattle, the vast majority of complex traits are under polygenic control, having many genes with small effects contributing to the variation of each trait, with some exceptions such as the diacylglcerol O-acyltransferase (DGAT1) [55] and ATP binding cassette subfamily G member 2 (ABCG2) [56] associated with milk production, myostatin associated with double muscle in beef cattle [57] and MHC DQ haplotypes associated with mastitis [39]. Here, ABCG2 is linked to mastitis.