Genetic mapping of dominant white (W), a homozygous lethal condition in the horse (Equus caballus)

Dominant white coat colour (W) is a depigmentation syndrome, known in miscellaneous species. When homozygous in the horse (similar in mice), the mutation responsible for the white phenotype is lethal in a very early stage of gestation. It seems, that the action of the dominant white allele is not always fully penetrant, resulting occasionally in spotted look alike offspring. These horses resemble a coat colour pattern known as sabino spotting. So far, it is not known whether dominant white (W) and sabino spotting (S) share a common genetic background. In this study, a pedigree consisting of 87 horses segregating for dominant white (W) was used to genetically localize the horse (W)‐locus. Microsatellite ASB23 was found linked to (W), which allowed us to map dominant white to a region on horse chromosome 3q22. Tyrosine kinase receptor (KIT) was previously mapped to this same chromosome region (3q21–22). KIT and its ligand (KITLG) are responsible for the normal function of melanogenesis, haematopoiesis and gametogenesis. So far, sequence analysis of different KIT gene fragments did not lead to new polymorphisms, except for a SNP detected in KIT intron 3 (KITSNPIn3). Additional microsatellites from ECA3q (TKY353 and LEX7), together with KITSNPIn3 allowed us to state more precisely the (W)‐mutation. The positional results and comparative functional data strongly suggest that KIT encodes for the horse (W)‐locus.