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Dominant White

The hunter mare pictured above appeared in Farm Livestock of Great Britain (R. Wallace, 1907). The caption notes that her white foal was sired by a dark chestnut stallion. The tendency of white-born horses to produce their own color 50% of the time lead early researchers to call the color “dominant white.”

Although the situation with this group of patterns – both in terms of inheritance and phenotype – is more complex than early geneticists expected, the name they coined is still used. Here is a partial list of instances where it has been used in peer-reviewed publications in the past three years.

From “Novel KIT variants for dominant white in the Australian horse population. (R. Hoban, K. Castle, et al., 2018)

To date 22 different alleles (W1–W19, W21–W24) underlying dominant white phenotypes have been identified.

From “A missense mutation in damage‐specific DNA binding protein 2 is a genetic risk factor for ocular squamous cell carcinoma in Belgian horses.” (K. Knickelbein, 2019)

This Belgian sample set showed genotypic variation in coat colour loci for agouti, dominant white 20 and dun.

From “Ten years of the horse reference genome: insights into equine biology, domestication and population dynamics in the post‐genome era.” (T. Raudsepp, 2018)

To date, 58 variants affecting pigmentation have been described, including 27 in the KIT gene that contribute to the dominant white phenotype.

The term has been used by researchers in recent articles in non-technical publications as well. It has not, as is sometimes asserted, been retired. Because it does have a long history of use when talking about white (and mostly-white) horses, and because it is widely used in other species, it is unlikely that it will disappear any time soon. I tend to use sabino as my umbrella term for this group of patterns, but others use dominant white or even both terms.