Monday, 28 April 2014

Dominant white, the most confusing coat colours

My erudite readers are no doubt saddened by my grammar in that title. They are right to be, of course. But you have to break eggs to make an omelette, and when it's time to describe a coat colour with one name (kind of, anyway) but over twenty known causes, consistent use of plurals is just one of the things that I'm prepared to sacrifice.

"The Opera House" shows the colouring expected from some types of dominant white horse: no pigment in the skin or hair, but normally pigmented eyes.

 "Dominant white"... Where to start? For one, this name is used to describe almost any mutation in one particular gene that results in white coat colour or markings. More than twenty mutations have been found so far. That gene is v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog. "Wow", I can hear you think, "what a snappy and memorable name for a gene, I understand what it does now". 

Just kidding. And let's just call it the KIT gene, that's what its mates call it. I'll explain just some of what it does later on.

Furthermore, what's with my sneaky use of "almost any mutation"?  Well, sabino 1 is a KIT gene mutation too. As far as I can tell, the only reason that sabino 1 isn't called dominant white is that it was discovered and named before the whole dominant white thing caught on. All of the things I say about dominant white apply to sabino 1, too. It has no special characteristics that differentiate it from the other types of dominant white, except perhaps for being quite common when the others are relatively rare. When someone says "sabino", think of it as being interchangeable with "dominant white". 

Marbrowell, another Thoroughbred showing markings that are probably dominant white.
From the first two pictures alone my readers have no doubt realised that there is lot of variation in the way dominant white can look. At one extreme are the completely white horses with brown eyes - Camarillo white horses are one example. There's no clear line at the other end of the spectrum though: one of the named dominant white mutations, W20, is described as only having a slight white-increasing effect.

This stallion, Dream Boy, is reported positive for one copy of W20.
So, given the lack of white on some dominant white horses, why is dominant white in horses called dominant white? I don't know if this is the only answer, but it appears to be because the first time scientists figured out a mutation that caused a complete lack of pigment in the hair and skin, but not the eyes (i.e. perfect whiteness but not albinism), it was for dominant white pigs that are completely white with brown eyes. That mutation was in the pig KIT gene. Since then, when a mutation is found in an animal KIT gene that causes a white coat or white markings, there's been a tendency to call it dominant white irrespective of how white it causes the animal to be or, for that matter, how completely dominant it is.

Dominant white pigs: pink skin, white hair and dark eyes.
Complete dominance is when having one copy of a gene is enough to produce the trait. For these pigs, the white colour shows complete dominance. But many versions of dominant white in horses are closer additive in their nature. Let's take sabino 1 as an example: one copy of sabino 1 gives a horse with high white socks, white facial markings, and maybe some extra belly splashes or roaning. Two copies of sabino 1 gives a horse that is 90-100% white: clearly a much greater effect than just one copy.

The pig thing also also seems to be the source of the original idea that inheriting two copies of a dominant white KIT mutation is lethal to the foal, i.e. that dominant white is homozygous lethal. This is also not necessarily true for equine dominant white. One obvious example is that it's not unusual to find horses that are homozygous for sabino 1, or for W20 for that matter. That said, it is likely that some of the more extreme horse KIT variants are indeed homozygous lethal. I am sure we will learn more over time.

Taking all this into account, it's not really surprising that many people are starting to just refer to the dominant white variants just plain old "white" instead. That's why they are called W1 to W20 etc, instead of DW1 to DW20 etc.

Supernatural, tested as carrying a combination of W20 and W5. This picture is linked through from the W20 Projekt, a site I wish I'd found earlier than the day before sample collection closed!
Not all equine KIT mutations that cause white markings have been found. For example, to the best of my knowledge the mutation that causes Clydesdale sabino markings is still unknown. Even outside of the heavy horse breeds and crosses, many people have their horse tested for sabino 1 and have it come back negative despite the fact that their horse has classic sabino 1 markings. If this is the case for your horse, have a look here at the Practical Horse Genetics dominant white exon sequencing (free until June 2014 for suitable horses).

So, what would you call your horse if it was positive for a KIT mutation? It doesn't exactly roll off the tongue but I think I'd say "she's bay W5 white", or "he's chestnut homozygous W20 white". And if the horse has two different KIT mutations, please remember that it's not homozygous white! Think of it as being like a horse with two different dilution mutations, like cream and champagne at the same time. That's a called a double dilute, so Supernatural up there is therefore probably best called a W5/W20 double white.

Still want to know more about the mechanics of dominant white? There is a more technical partner blog entry to this one coming right up.

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