OK, I may have been mistaken, or at least the answer is unknown. Of interest: PLoS Genetics: Allelic Heterogeneity at the Equine KIT Locus in Dominant White (W) Horses
A complete loss of function of KIT causes prenatal or perinatal
lethality due to anemia.
regulatory mutations exist that produce severe pigmentation phenotypes in the heterozygous state. However, some of the murine structural mutations, such as the KitW-42J
mutation, also lead to an almost complete depigmentation in the heterozygous state. In the mouse, mutations causing a pronounced dominant depigmentation phenotype typically also lead to mild anemia and reduced male fertility even in the heterozygous state.
In horses, dominant white was reported to cause embryonic lethality in the homozygous state [4
]. However, this report on the embryonic lethality was derived from the analysis of offspring phenotype ratios in a single herd segregating one or more unknown mutations. As there is now evidence for allelic heterogeneity, it remains to be proven whether all equine dominant white mutations cause embryonic lethality in the homozygous state. While this is certainly likely for the two nonsense mutations found in Franches-Montagnes Horses and Arabians, it should not necessarily be assumed for the two reported missense mutations or for any of the other as-yet unknown W