Dominant white and white spotting

What it looks like

Despite the name, dominant white does not always produce a completely white horse. Some variants give extensive white with pink skin; others are associated with more modest facial or leg markings. A horse born white differs from one that gradually turns gray. Photographs of adults can make it difficult to tell those histories apart.

Sources for this explanation [2]

What happens biologically

KIT is a gene containing instructions for a receptor, a protein that receives signals needed for pigment cells to develop and survive. Changes in its DNA can leave areas without pigment. Many separate variants have arisen in different families and breeds. W5, W13 and W20 name particular versions, or alleles, rather than strengths of one universal white allele.

Dominant means that one copy can affect appearance. It does not mean that every horse carrying a W variant must be entirely white. Other pigmentation variants can modify how much white is visible.

Sources for this explanation [3] · [4]

Inheritance and DNA testing

A horse inherits one chromosome carrying KIT from each parent. If it has one copy of a particular W variant, it can pass that copy to about half its offspring. For example, with a one-copy W20 parent and a parent tested without W20, each conception has a 50% chance of receiving W20. This predicts transmission of that variant, not the size or shape of the foal's markings.

A panel may test only selected known W variants. Several patterns also involve the KIT region: nearby variants can travel together on the same chromosome, a situation called genetic linkage. Their arrangement matters, so one cannot simply multiply separate probabilities as if every white variant were inherited independently.

Sources for this explanation [2]

Confusions, limits and health

Survival must be considered for each specific allele. Homozygous means having two copies of the same variant. For some KIT variants, the absence of observed homozygotes and molecular findings suggest that two copies may prevent an embryo from surviving; that is not a rule for all W alleles. W20 homozygotes can be viable. This differs from the EDNRB syndrome, in which affected white foals can be born alive and then die from intestinal dysfunction. A negative partial W panel does not exclude an untested KIT variant.

Sources for this explanation [2] · [1]

Where the explanations come from

Laboratory resources explain which variants are tested and how to read their results. Research papers describe the evidence, discoveries and remaining uncertainties. Each article links its explanations to the relevant sources. Published titles stay in their original language so you can find the exact work.

1. 1 · Research paper

   Santschi et al. (1998) — Endothelin receptor B polymorphism associated with lethal white foal syndrome in horses

2. 2 · Laboratory explanation

   UC Davis VGL — Dominant White Mutations W5, W10, W13, W20 and W22

3. 3 · Research paper

   Haase et al. (2007) — Allelic heterogeneity at the equine KIT locus in dominant white horses

4. 4 · Research paper

   Haase et al. (2009) — Seven novel KIT mutations in horses with white coat colour phenotypes