If you’ve had a Samoyed for any length of time, you have heard of the mythical black hair. Maybe you’ve even found one or two on your own dog! If you’ve read my previous article on why Samoyeds are white, you will know that Samoyeds all possess two copies of the recessive red gene. This gene prevents Eumelanin (black, brown, blue, and lilac) pigment from expressing in the coat. Since Eumelanin is blocked, only Pheomelanin (red pigment, or in our case, white) colors the coat. So black hairs on a recessive red dog should be impossible, yet here they are! What’s going on? They are somatic mutations!
What is a Somatic Mutation?
Somatic mutations occur during fetal development when cells are still splitting and multiplying. A somatic mutation is an error in the cell division process. The resulting cells have a broken or altered version of a gene. This broken copy then spreads to all cells that divide from the original pair that the error occurred in. This definition applies when this error happens in any part of the DNA, but for the purpose of this post, we are only looking at the E Locus. All somatic mutations discussed after this point are specifically E Locus somatic mutations. Recall from my previous posts, a locus is a specific location on the DNA strand, and each locus contains two alleles (genes).
So Which Gene Broke?
All Samoyeds are homozygous for the recessive red gene (notated as being e/e, or possessing two ‘e’ alleles). Recessive red is itself an E locus mutation, a deviation from the wild type (the “default” allele at this locus) which allows for regular Eumelanin expression. Out of all of the mutations present in dogs today, recessive red seems to be an especially fickle one. It’s more prone to breaking than other genes. When ‘e’ breaks, Eumelanin production is no longer restricted. It effectively turns back into ‘E’, the dominant wild type allele on the E locus. Since two ‘e’ alleles are required for the coat to be white, all cells in the coat where it has broken will show whatever coat color the dog would have been if it were E/e instead of e/e.
You can see the steps involved in figuring out what this color would be in my previous post: What Color Would Samoyeds be if They Weren’t White?
Most Samoyeds are Black
For Samoyeds, the vast majority of dogs are black underneath the recessive red. So whenever a somatic mutation occurs in the coat, black hair will appear. How large the black area is depends on when in development the somatic mutation occurred. The earlier it happened, the larger the black patch. Most occur later and only encompass a few hairs, but there are a handful of viral Samoyeds with larger mutations. Most notably, Niki with her black ear “beret”. There is also a Samoyed with a black tail, and every now and again in facebook breed groups, you’ll see a puppy with a coin-sized black spot. I’ve yet to come across a more significant mutation than Niki’s, but we know that there is almost no limit to how extensive they can be thanks to looking at other recessive red dogs like Golden Retrievers and Yellow Labs. The reason we don’t see as many (or any, as far as I’ve seen) significantly sized somatic mutations in Samoyeds is likely because, unlike their Golden and Lab counterparts, Samoyeds are also largely piebald (white spotted). Since white spotting covers the e/e (even though they’re both white so you can’t tell most of the time), you won’t get somatic mutations anywhere that the coat is white because of piebald instead of white because of e/e.
Are Somatic Mutations Inheritable?
We are all thinking the same thing: Can somatic mutations create a new variety of black Samoyeds? The answer, fortunately or unfortunately, is no. Early on in the development of an embryo, cells split into two groups: Somatic and Germline. Germline cells go on to become eggs and sperm, while somatic cells make up the rest of the body. So purely by definition, somatic mutations are uninheritable because they occur in somatic cells. A germline mutation, while possible, is extremely unlikely. Coupled with the fact that dogs with visible mutations are often fixed and sold to strictly pet homes, there is immense selective pressure against our mythical black Samoyed. If you do find yourself face to face with a suspiciously not-white Samoyed, most likely it’s a Samoyed Mix.
How do we know the Black is a Mutation and not just another Coat Pattern?
Though it may not seem like it, there are actually only a handful of coat colors and patterns a dog may have. As fun as it is to imagine, new and inheritable mutations just are not that common. The pattern of black seen in our example dogs above is vastly different from any of the regular coat patterns we see. The other giveaway is the jagged, almost glitch-like edges the larger mutations seem to have. Some people mistake the dogs with larger mutations for Chimeras (a single animal with two distinct sets of DNA). However, chimeras are exceptionally rare, and very few dogs are DNA confirmed to be chimeras. Most viral “Chimera” dogs and cats are actually just somatic mutations. In the case of cats, it’s usually just a unique “split face” presentation of tortoiseshell patterning. Merle dogs are also commonly seen with this split face phenomenon.
What other Somatic Mutations are there?
While E locus somatic mutations seem to be the most common, it is not the only locus known to spontaneously mutate. Rarely, you will find a dog with a B or D locus mutation. The B locus controls if a dog is black-based or brown-based. The D locus controls if a dog is dilute or not. The dilute gene will turn a black dog into a blue one or a brown dog into a lilac one. There are some oddities out there with white spotting, but there are so many untestable white spotting alleles it is unclear whether these oddities are mutations or just other alleles. But perhaps the most common somatic mutation of all time is Cancer. Cancer occurs when a cell mutates into being immortal. This list is by no means exhaustive, but you get the idea.
Extra Reading
For more information on Somatic Mutations and real (non-Samoyed) examples, check out this article by DogGenetics.co.uk
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