It's unknown what causes it. My first thought was to investigate hairlessness in other animals. So I did a Google search. I have an interest in equine color genetics and I know that the same genes present in the horse are often in other animals as well. The same basic red-or-black base coat, dilution genes, and pattern genes seem to exist in a wide variety of mammals.

So, I searched for the history of hairless cats and dogs. I found out that the first hairless cats were 'freaks' that were from normal-haired parents. People bred these cats to form a breed, the Sphynx, but every now & then a hairless cat will still randomly pop up from normal parents. I've read that the hairlessness in the cat is produced by an allele of the same gene that produces a curly haired coat of the Devon Rex (curly-haired breed of cat). Both the curly allele and the hairless allele are recessive to the 'normal' allele, which is why the hairlessness can seem to randomly pop up when two carriers are bred.
This is also interesting to note because a curly-haired gene exists in the horse as well! (the Bashkir Curly horse for example). I wonder if the curly-haired allele in the horse corresponds to that in the cat, and if the hairlessness in the horse is caused by a rare allele of that gene, like in the cat.

In dogs, hairlessness is also recessive, and can appear to pop up from nowhere. Animals can carry it for many generations, and unless two with recessive genes are mated, you would never know. Dogs, like cats and horses, also carry a curly-hair gene.

It is interesting to note that, like the Bashkir Curly horse, Devon Rex cats, and several curly-haired dogs (Poodles, Schnauzers, Bichon Frise) are often hypoallergenic.

In the article about Harry, the hairless Percheron, it notes that his parents were 'well matched' and some people suspected were brother & sister. While mating close relatives won't normally cause complications itself, this definitely could have contributed if both parents carried the recessive hairless gene (that is, if this gene even exists in the horse as I'm speculating).

I'm guessing that hairlessness in the horse is probably caused by an extremely rare recessive allele of the normal/curly gene (which we know is present in the horse). It is probably similar to what occurs in the cat and dog. From what I have read no one else has any theories or speculations on the cause of it, but if I find out more I'll post it here.

If you have any information or comments, please send me an email: webmaster@ultimatehorsesite.com. Thanks!

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