I know I said I’d post about the Dominant White, but I wanted to share this photo sent by my friend Jackie Arns. This is a rescued pit bull that Jackie saw as a patient. Much like Domino, the Cocker in the previous post, she has a high percentage of color, but she had the abnormal pupils of a double merle. As these pictures show, she has merling that effects her red pigment. (By red I mean the pigment color as horse people understand it, not the color many dog people call red but that is the result of the brown (b) gene. Dog people would call the color “yellow” rather than red.)
These are great photos of what I thought I had seen in a few other dogs. I have seen speculation that the presence of sable allows merle to act on the red pigment, though I am not aware of any formal research on this. I also do not know if it is possible to find a clear, non-sabled red (yellow) dog with visible merling. It is also possible that the fact that this particular dog is homozygous plays a role in the merling of the red pigment. This gal is a sable, though she only has a bit of smudging on her ears and face.
The question then is why do dogs like this pit bull show merling in the red areas, while Lily (the sable merle Collie from the first post) and most others do not?
Another friend, Cindy Dalton, included this link in the comments. The dog, a Border Collie named Psycho, does have reddish patches on what looks like an extremely dark liver (recessive brown, b) background. He has the paler, redder patches because he is what is called a tweed merle. That is the other modified form of merle besides harlequin. Where harlequin strips out the silver coloring, making the merled areas white, tweed takes the dark areas of the merle and dilutes them to a different tone – or a lot of different tones. In some dogs the diluted patches are rather reddish, while in others they look more pewter or slate. The effect on Psycho is easy to see in this picture. (His cool parti-colored eyes can be seen in this one.)
My Aussie mix, Emma, is a tweed merle. In her case the diluted patches range from a slate gray to a pale brownish gray. The largest one, which runs down her chest to her upper right leg, is visible in the picture I posted of her earlier. The same area on the other side is diluted, though the patches are intermixed with true black rather than being one large area. Her lower legs and muzzle all have smaller dilute patches, which give her legs and face a warmer tone than the colder silver on her back.
Here are close-ups of the areas that shows the two different tones really well. Because the warmer dilute patches were concentrated on her legs and her muzzle, I originally wondered if she might be some odd black and tan color under her merling, but the black and tan patterns have red (yellow) pigment trimming the black. Sometimes the tan (red) pigment is really pale, almost white, but that isn’t what she is. Her lighter areas are quite obviously a dilute of black pigment.
Tweed was first identified in Aussies and is thought to be dominant. What isn’t clear is whether all the variations are the same gene. Some merles just have a few diluted spots. Others have many patches of many different colors. Still others have white patches mixed among the colored ones. Those are often called harlequins – a term not especially helpful since it is already used for the modifier found in Danes. They might all be tweed, or they might have different genetic causes. There was an initial study done by Dr. Sponenberg on tweed in 1995, but I don’t think any further ones have been done.
And for those with a more technical bent, the original study that identified merle is available here. (As technical papers go, it is very clearly written.) The study that located the (Great Dane) harlequin gene is available here.