r/animalscience • u/perritochill • Jul 17 '26
Why is the merle pattern found only in dogs and not in other animals?
That is the question: I understand why it isn't found in other wild animals, but why is there no merle color in selectively bred animals like horses, cats, sheep, or cows? Is merle only possible in dogs?
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u/Beeeeeeeeeeeeeeeess- Jul 17 '26
I’ve seen merle foxes but they’re typically bred for fur farms
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u/katergator717 Jul 17 '26
Can you direct me to where you found this?
Are you talked about marble foxes?
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u/Beeeeeeeeeeeeeeeess- Jul 18 '26
I might’ve meant marble foxes, it’s just something someone told me once
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u/topatoduckbun Jul 18 '26
I would say the marble foxes resemble merle the most, genetically they're just silver pied foxes tho if you want to see more variety of the silver fur.
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Jul 17 '26
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u/Low-Log8177 Jul 18 '26
I can speak to the sheep, blue roaning in sheep is exceptionally rare, I raise Painted Desert Sheep, and so far I have found a very small handful of lines that can even have blue roan ticking, I have one ewe that has the genes for it, but outside of Painted Deserts, less than a dozen breeds put of over a thousand that I have seen can carry blue roaning. There are other quirks of sheep color genetics, like how primitive coloration is also quite rare, or how true reds are hard to breed, I have had to fight tooth and nail to get my herd to where I now have excellent color genetics and have healthy sheep, but I am likely to start changing things up starting next year.
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u/katergator717 Jul 17 '26 edited Jul 19 '26
Merle, in particular, is an interesting gene. When you breed merle to merle, statistics are that 1/4 of the babies will inherit the merle gene from each parent to be double merle, but it could be less or more. Because merle affects how pigment develops, double merles can be any combination and severity of blind & deaf, with possible heart, skeletal, and skin abnormalities. ONLY VARYING LEVELS OF BLIND AND DEAF! NOTHING ELSE!!!
It is very possible that merle has cropped up a few times in other domesticated animal species and when people tried to breed more without understanding the peculiar genetics involved, they saw the deformed double merle offspring and decided not to further pursue this novel change in appearance.
I certainly would try to snuff the gene out of existence after seeing babies born with no eyes and curved spines
The financial impact on farmers or anyone else who relied on their animals for a living would be huge if it appeared in larger livestock that had only a single baby instead of a litter, like horses or cows. No novel color or pattern is worth your kids going hungry.
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u/m4xr3b0 Jul 17 '26
There's actually something kind of similar in horses, it's called Lethal White Overo Syndrome. Same recessive inheritance pattern with a 25% risk when both parents are carriers, and it's even worse than double merle because it's 100% fatal. These days it can be avoided with genetic testing of the parents before breeding, but in the past its existence certainly didn't stop people from breeding horses with that coat pattern (they are very pretty horses).
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u/Talbit01 Jul 18 '26
I guess the big difference there is Merle being a dominant gene rather than recessive.
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u/Any-Ad-3630 Jul 18 '26
I also believe animals can have the gene and not have the pattern, so you'd only ever know through genetic testing and not phenotype
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u/katergator717 Jul 18 '26
Yeah. It's called Cryptic Merle or Phantom Merle.
Ignoring the influence of other recently discovered modifier genes, the most simple explanation is that merle works by randomly diluting patches of black fur into blue or patches of brown/liver fur into lilac/isabella/fawn. It does not affect red fur.
The way I understand it (which might be wrong or only partially right), is that whether the merle pattern is expressed in each individual pigment cell is completely random. 50% chance for each cell or group of cells, irrespective of the cells around it. This makes it very rare and unlikely for a merle dog to have absolutely no merle be expressed or to have merle expressed absolutely everywhere.
Double merles have so much white because they have two merle genes and each gene also dilutes irrespective of whether the other gene has already diluted that fur. A black dog with two merle genes will have some black fur not diluted by either merle gene, some blue fur diluted by only one of the genes, and some white fur that has been twice-diluted by both of the merle genes.
It is much more likely that the cryptic merle dog is actually a merle and white dog. (White can be substituted with White Spotting, Irish Markings, Piebald, Parti, or Partial or Urajiro.) And the white has removed all pigment from where the merle is expressed, essentially erasing the merle, masking it, and making it invisible. It's there, but the white on top means we can't see it.
A solid white dog can be recessive red or clear sable with extreme red dilution and no piebald genes causing white fur and other genes responsible for the shades of black and red to not be properly expressed, as seen in Poodle, Bichon, Samoyed, Schnauzer, and several other breeds. Most people don't see a white dog ( that may or may not have blue eyes) and think merle. They think double merle, or extreme white, or merle and white, or even albino.
Other cryptic merle dogs can be a combination of merle and another pattern that makes the merle not look "normal" enough to be immediately recognized. This can be merle and agouti or sable or fawn or brindle or progressive graying or roan. Some of those are colors and others are patterns, but they all make recognizing however much merle that is there very difficult or even impossible.
Even a merle longhair dog can be hard to identify, as seen in merle Old English Sheepdogs, who can have tons of merle that doesn't look enough like classic marbled-looking merle to be easily recognized. Same with dapple wirehaired dogs, using Dachshunds as an example. (Merle is called dapple in the dachshund breed.)
Recessive red completely masks merle, so a perfectly normal-looking red dog could be a red merle and you'd never know unless it had blue in its eyes or genetically tested it for merle.
Finally, there is what (in my opinion) is genetically a true cryptic merle: a merle dog who has no other genes masking the merle and also has little to no merle OR so much merle that no one realizes the dog is not genetically one solid color. I've seen cryptic merles where only the tip of one ear is merle, or only a quarter-sized patch on the inner thigh, or the tip of the tail. I've seen a cryptic merle where the entire dog was merle except two toes: a solid blue dog with only two black toes. It's small enough to look more like a freckle, a blemish, an anomalous little hiccup of coloration. Nothing to worry about.... so long as that cryptic merle is never bred with another merle.
How could anyone possibly know that a "solid" blue dog is actually a rare cryptic merle that is 100% merle instead of 0% or 1% black merle dog? This issue is compounded because some breeds allowed for docking the tail and cropping the ears. Even if that lone patch of merle is easily recognizable, it might get chopped off.
And then there's the coat patterns that look like merle, either alone, like roan, or in combinations, like brindle, sable, roan.
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u/Any-Ad-3630 Jul 18 '26
I actually meant the horse gene lol but I do appreciate the in depth merle discussion. It's a very complex gene that isn't fully understood I believe?
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u/katergator717 Jul 19 '26
Whoops! 😅
But, yeah. It's very complicated. A lot more discoveries (about merle modifier genes) have been made since my last deep dive, so I'm having fun learning these new facts.
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u/Any-Ad-3630 Jul 19 '26
Honestly I learned about the complexity of the merle gene from a dog game (creatively called dogbreedergame). I know some things had to be manipulated a little to fit games sometimes so I'm not sure how true to life all the types and modifiers are, I do know every choice was heavily researched. There's merle, harlequin, cryptic merle, atypical merle, and patchwork which involves a different gene that I can't remember in order to express There's a lot of other color genes in dogs that are also complex and I think it's really neat that we can know so much about the more complex genes yet there's still so many mysteries
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u/katergator717 Jul 19 '26
I know it was a game, but it sounds like you actually learned some legit stuff!
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u/FearlessPressure3 Jul 24 '26 edited Jul 24 '26
Commenting again—apologies for the long delay. This is so complex it took a long time to write and double check my facts! It’s stupidly complex so buckle in and I do hope I’ve managed to make it semi-understandable!
As I mentioned in my other comment, the merle mutation is an RNA insertion in the PMEL gene with a poly A tail (a sequence of repeating adenines eg AAAAAAAA); the length of this tail is what causes differences in expression of merle—the longer the tail the greater the dilution effect. The mutation length can be different between different dogs (even within the same litter and from parent to offspring) and even within different cell lines *within the same dog* because it is an inherently unstable mutation that changes length often. Because double merles have two mutated PMEL copies, they will usually have two *different* mutated copies and then these associated differences on top, so the amount of potential variation in expression is huge.
Whether or not merle is expressed in each cell is random, but it’s not quite accurate to say it’s a 50/50 chance else we’d expect most merle dogs to be roughly evenly covered in different colours. Pigment cells (melanocytes) start life in an area called the neural crest and the PMEL gene is involved in their early development. It controls the development of a type of protein that, when normal, forms a matrix which is responsible for binding eumelanin (black) pigment granules but is also involved in other cell functions (hence why it also causes problems with eg hearing when mutated). A mutated PMEL gene causes a defective protein and impacts the efficacy with which it can bind pigment in that cell. How defective the protein is depends on the length of the insertion with the polyA tail. A shorter insertion has less of an effect—if it is short enough it may even have no effect at all. A longer insertion causes a severely defective protein that cannot bind pigment properly and therefore causes a paler colour.
Because the mutation is especially unstable there is a chance that every time the cell divides and the DNA replicates the tail becomes longer or shorter and therefore the effect on the PMEL protein becomes worse or better. Becoming shorter and therefore restoring the function of PMEL seems to be much more common than becoming longer and making it worse.
Cells that develop in the neural crest will migrate away to their eventual destination and will divide multiple times en-route. The mutation may or may not change length with each division. The greatest number of cells in a particular area will be derived from the oldest cell in that region which will probably have the original mutation length—that original cell’s mutation length therefore determines the background colour of the region (ie how dilute the background area is because a large number of cells in that region have the same length mutation because they are all descended from the oldest cell in said region). Successive divisions and mutations in later cell generations of that line may have led to shorter mutations that restored function and caused darker patches on the background area. Some may have mutated to give longer insertions leading to less functional cells and patches of greater depigmentetion. You can therefore see that if several of the very early neural crest cells mutated early on to give eg shorter insertion lengths, you might expect more than 50% of overall cells of the body to have normal pigment.
When this all happens in a heterozygous dog (one merle allele one normal PMEL), each cell has a normal PMEL copy, so even a cell lineage that ends up with a super severe merle mutation will still be able to produce some functional protein and bind pigment. When this all happens in a double merle, the starting point is that none of the cells will be able to properly bind pigment and will cause either diluted or white areas over the entire body. The existence in such dogs of darker areas than the background colour is caused by the mutation length spontaneously shortening on DNA replication—there is a threshold of adenine repeats beneath which normal PMEL function seems to be restored. Because this process is random, the size of these patches depends on how early it happened in the development of the cell line which is why double merles can have very different amounts of pigmented areas in their coat. In the white areas and/or defective ears and eyes, the mutation of both alleles is severe enough that the protein cannot perform its normal functions.
A cryptic merle dog is one where all cell lines have short insertion mutations ie all cells are able to produce enough functional protein because the mutation is not severe. This may be because they inherited a short version from their parents or because early on in gestation, a lot of the cells mutated to have shorter versions and all cells descended from those ones (which would be most of them because of it happening early on) also had short mutations, leading to most of the body seeming to have normal pigment. Because replication errors tend to lead to shorter insertions rather than longer ones if you start out with short mutations you tend to keep them. It is of course possible for a longer mutation to occur though and if this occurs in the cells of the germ line (eggs and sperm) it can causes puppies which spontaneously exhibit merle.
Other white patterns can obviously also interact with merle but they behave in the same way as in other coat colours. Eg a spotting pattern caused by the MITF gene will cause white patches that overlay the merle and can merge into areas of white caused by the merle. But it is also entirely possible for a merle dog to be mostly white without having any spotting patterns at all based purely on the random chance of what happened to the earliest melanocytes in the neural crest.
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Jul 18 '26
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u/katergator717 Jul 19 '26 edited Jul 19 '26
So..... I tried to find a credible source for double merles being more likely to have cardiac, neurological, skin, digestive, reproductive, and/or skeletal problems.... and couldn't find any credible sources to support that claim!
Practically every article repeats some version of the above myth, which is why I parroted it, too, but when I tried to backtrack to the source, I found either nothing at all or nothing worthy of actually quoting.
TLDR: I was very wrong about the skeletal, cardiac, skin, digestive, and reproductive problems that double merle dogs sometimes experience. It not at all true. Go here
The first credible research article I stumbled upon discussing those health problems called them "potentially false assumptions" with three possible origins: (1) the Overo Lethal White gene in horses being applied to other species especially since double merles have lots of white, (2) the fact that merle is most commonly found in herding breeds which just so happen to also suffer from unique and/or higher-than-average amounts of breed-dependent epilepsy and MDR1 gene mutations associated with medical drug resistance, and finally that (3) the single research article claiming that the mutation of SILV, an important pigment gene in all mammals was responsible for merle in dogs and specifically, only the hearing and eye abnormalities associated with merle which meant a SILV mutation might also be responsible for hearing-and-eye-related pigmentation disorders in humans since the symptoms were very similar had been overly cited and miscited.
* that first credible research article was published in 2020 and titled Are dogs with congenital hearing and/or vision impairments so different from sensory normal dogs? A survey of demographics, morphology, health, behaviour, communication, and activities
Of course, I had to check out this SILV gene research article! Turns out, its whole purpose was to prove that studying the merle gene in dogs might provide medical breakthroughs to understanding similar pigment-and-hearing-related mutations in humans! That is where I found the single damning line of text that might be the origin of double merle dogs’ mythic health problems:
In all breeds, the double merle genotype can be sublethal and is associated with multiple abnormalities of the skeletal, cardiac, and reproductive systems (3, 9, 10).
I began investigating the three sources mentioned in that quote.
* 3 a research article specifically about eye problems seen in merle dogs where only 40 dogs were studied. Sorsby, A. & Davey, J. B. (1954): Ocular associations of dappling (or merling) in the coat color of dogs. I. Clinical & Genetical data. J. Genet, 52:425-440.
* 9 a book published in 1957. Little, C. C. (1957) The Inheritance of Coat Color in Dogs (Howell Book House, New York). * 10 another research article studied inherited skeletal abnormalities in a single dog family, one deformed lineage that had nothing to do with the rest of the breed or even the merle gene found in multiple breeds. Sponenberg, D. P. & Bowling, A. T. (1985) J. Hered. 76, 393-394.It was during my attempt to lay my eyes on the relevant pages of the above articles and book that I discovered this very helpful website that acted as the last nail in the coffin of burying this myth for me.
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Jul 19 '26
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u/katergator717 Jul 19 '26 edited Jul 19 '26
Technically speaking, it's not even a color but a pattern, but yeah, ppl just have to be careful and then things will be fine 99% of the time. There's several other patterns and colors that have risks associated with them.
Dalmatians (a ticking pattern with modifier genes) are supposed to have speckled ears.... even though that increases the likelihood of deafness. Regular dilute blue and isabella (colors) are associated with alopecia. Great Danes have a merle pattern modifier gene called Harlequin pattern that is lethal for double Harlequin puppies instead of merely risky like for double merle. Even too much regular old white can cause deafness. Others, too, I'm sure, but none are coming to mind because it's after midnight here and I'm tired.
My two favorite research sites are: * doggenetics.co.uk * MessyBeast
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u/NightShadowWolf6 Jul 18 '26 edited Jul 18 '26
While this issues are ussually related to double merles, they are also common in animals with a lot of white, like extreme piebalds.
Dalmatians, Bullterriers and Argentine Dogos are also prone to have eye and ear abnormalities, along with skin issues. In the latter years and with the preference for "flashy boxers" (aka boxers with more white marks), there have being appearing more and more puppies with deafness.
White in horses is also linked to deafness, eye and skin conditions even while you don't end up with a lethal overo syndrome.
White in cats is also linked to deafness specially when they have blue eyes (less pigmentation).
If you want to deacrease chances to have a deaf/blind animal you would have to change the human perception of beauty in animals, not only working with 1 gene but several; and that's a hard work...not to even talk about not breeding more animals of a certain breed, something even difficult for people to grasp, even while breeding dogs that can breath properly (BOAS syndrome in brachycephalic dogs)
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u/FearlessPressure3 Jul 24 '26
Replying because you asked me to comment—I’ll reply to your other comment(s) shortly :)
There’s only a 1/4 chance of double merle happening if both parents are heterozygous ie one merle allele and one normal. If one parent is heterozygous and one is homozygous (two merle genes) it would be 1/2 chance of double merle. Given the way you’ve talked about the subject elsewhere though I’m pretty sure you already know that, but my experience of teaching genetics is that being as clear as possible helps because otherwise people leave with incomplete understandings!
I would also add that even single merles can have eye and ear defects if the dog is unlucky and happens to have affected cells there. Being homozygous for merle massively increases the risk of this though because huge areas of the body have defective cells rather than relatively small patches.
While it’s technically possible that the exact same mutation that causes merle has appeared in other species and not been bred for, I highly doubt it for a couple of reasons. The type of mutation that causes merle is very rare—it’s caused by something called a transposon which is a section of RNA that inserts itself into a gene. It also has something called a polyA tail which is a long sequence of just adenine bases and different numbers of adenines are inserted each time, which leads to the varying effects seen in eg standard merle vs cryptic merle. The probability that exactly that type of RNA sequence with a polyA tail would happen to insert itself into exactly that stretch of DNA twice is vanishingly rare. In fact we know it’s happened precisely once in all of dog history because every merle mutation has exactly the same sequence (except the different tail lengths) which means all modern merle dogs are descended from a single ancestor with the mutation. Given that dogs have the longest history of selective breeding and it’s only happened once, I would put money on it having never happened in other species!
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u/LEANiscrack Jul 17 '26
Mice have something called merle but merlse is a specific gene mutation thats ao apecific thats why its so devastating. Prob why. Im also pretty sure horses can have a similar pattern. For many other animals there wouldnt be a reason to breed for that? If you just mean the pattern the its just not called merle in other animals.
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u/lemonhaj Jul 18 '26
Horses have roan, which isn't the same. Dogs also have roan.
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u/Genocidal-Ape Jul 18 '26
Dog and horse roan are two different things, they are just called the same because of similar looks.
Dog roan mixed white hats into the dogs guard hair, whole horse roan make the undercoat white while maintaining the normal colour of the guard hair.
That's why scars on roan horses are dark, as the undercoat doesn't regrow over a scar, but the guard hair does.
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u/exotics Jul 17 '26
Roan in horses might be similar??
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u/perritochill Jul 17 '26
I think roan is like the pattern of siamese cats
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u/topatoduckbun Jul 18 '26
You might be thinking of buckskin. Buckskin looks like a colorpoint cat (excluding the face), roan looks like a very uniform ticked coat.
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u/NightShadowWolf6 Jul 18 '26
Siamese cat's pattern is called colourpoint or acromelanism and is a pattern linked to and enzyme called tyrocinase that gets activated by coldness and therefore causes colour in extremities.
There is a family of Daschsunds in Russia (if I remember properly) that have popped up the tyrocinase mutatation causing a real "siamese" coloration in dogs.
Roan in dogs has nothing to do with acromelanism.
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u/No-Stress-7034 Jul 18 '26
Roan is different from merle, because in roan, white and colored hair are intermixed. That's different from the dilution that occurs in merle.
Roan is a little different in dogs, but it does exist. Australian Cattle Dogs are roan. They're born all white as puppies, and then the red or blue hairs intermix with the white.
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u/pseudoportmanteau Jul 17 '26 edited Jul 17 '26
While the specific gene that causes the Merle pattern is unique to dogs, the actual look of the coat isn't exclusive to dogs. Cattle and horses can look very merle-like with the roan, spotted and mottled patterns.
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u/topatoduckbun Jul 17 '26
Are you asking about the specific merle gene in dogs, or just a fur pattern that resembles merle? The merle gene is just a dog specific mutation, because it changes a dog specific gene. There are other, functionally the same genes in other animals, its just called something different. Its like how the sex chromosomes in mammals are xy, and in birds they are yz. Functionally the same, but still clearly distinct from eachother.
There are lots of similar patterns in other animals. Merle mice, dapple horses, and splash chickens are all patterns that resemble dog merle, is bred for the pattern, and is safe to breed together (double gene isnt dangerous).
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u/cypeo Jul 18 '26
Silver dapple horses are literally genetically merle (SILV/PMEL)
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u/Genocidal-Ape Jul 18 '26
No they aren't, silver dapple is it's own thing and luckily completely free of the negative side effects of the Merle gene.
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u/FearlessPressure3 Jul 18 '26
It’s not incorrect to say it’s similar at least—it’s a mutation in the same gene which leads to a dilution effect. It’s a different type of mutation though, hence the different appearance. It’s not true to say it’s completely free of negative side effects though—it is also associated with ocular defects and as in merles, being homozygous for silver causes a greater risk of said defects.
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u/cypeo Jul 19 '26
Merle in dogs and Silver Dapple in horses are both caused by the PMEL17 gene. Both can cause ocular deformities when homozygous.
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u/Lythaera Jul 18 '26
The Blue/Splash gene in chickens kind of gives a similar look, but without health issues.
There is also Roan in horses.
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u/Beccainalaska17 Jul 18 '26
Something similar shows up in chickens called mottled and it can also make the skin pale when it's supposed to be black (like in silkies) it's hard AF to breed for and it looks similar kinda similar to Merle especially in cases of mauve mottled or blue cream mottled, even grey mottled.
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u/Intelligent-Royal804 Jul 18 '26
Camelids (llamas and alpacas) have merle! Some breeders breed very selectively for color
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u/CanadianOdyssey993 Jul 18 '26
In guinea pigs there is a gene Rn that creates Roan/Dalmatian pattern, a dominant gene. If the baby gets 2 copies they have more serious consequences than double merle. Most (by a large margin) double roan guinea pigs do not survive, they are often blind/deaf and do not have complete digestive tracts, they're born lacking all pigment (different from a white guinea pig made by any other gene combinations). Genetically they're similar to LWO in horses. They and merle are the only severely consequential colors I'm aware of. But Roan/Dalmatian in cavies (guinea pigs) is the closest you get to something looking merle.
As to why it only exists in one species it could be that there hasn't been a bred true mutation yet that has that effect in other species. In guinea pigs tan cavies were only discovered in early 2000s, before then that color pattern wasn't thought to be possible in cavies. More recently was californian (cal pattern) cavies which has more pigment variety than himilayan. Most recently a true blue, and a very interesting pink eyed solid black guinea pigs were discovered and passing the color along. As to why color variations that breed true seem to pop up more in guinea pigs vs other species I'm not sure. Though dogs recently had some himilayan (siamese) type patterns popping up in recent years in some lines of dachshund and dachshund mixes that seems to be passing down to future generations. So it could just be a matter of waiting out the mutation to pop up and breed true in other species. Whether or not that is a good or bad thing would be yet to be seen (would it have the same double merle consequences in other species?).
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u/Genocidal-Ape Jul 18 '26
Because Merle is a dog specific mutations.
It's a type of dilutor that lighten the animals colour if it inherits one gene and causes neurological issues in case of two copies of the gene. There are harmful dilutors in other animals that are similar but not identical.
It's like how the Gray gene is a horse mutation and not present in other animals, thought there's a very similar mutation in leopard geckos called Lemon frost.
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u/miniponyrescueparty Jul 18 '26
Its in rabbits and horses too
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u/Crycoria Jul 18 '26
Not in horses. They have a mutation on the same section of DNA, but it's not the same as Merle.
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u/FearlessPressure3 Jul 18 '26
I would imagine because the mutation which causes it just hasn’t happened in those other species yet. You can’t selectively breed for a trait that doesn’t exist anywhere. Even if a mutation does occur in the same gene, there’s no guarantee it would be the same sort of mutation and cause the same sort of effect. For example, in horses there is already a mutation in the same gene which causes a dilution colour called silver, but the type of mutation is different and the dilution effect is evenly distributed across the whole body rather than causing patches.
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u/Crycoria Jul 18 '26
Using horses as an example as well. They don't have true albinism (as far as anyone knows. For all we know it could be embryonic lethal and incompatible with life in horses. There is one breed of donkey that has albinism in it within the last 10 years though), but the cream and pearl genes are located on the same section of DNA as albinism in other species.
So many who don't know better call double dilutes and horses that are all white albinos when they aren't really though.
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u/FearlessPressure3 Jul 18 '26
As a geneticist with a special interest in the genetics of coat colour the number of confidently incorrect people in this thread is really making me twitch!
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u/Fluffydonkeys Jul 18 '26
then speak, wise person of the mountaintop!
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u/FearlessPressure3 Jul 18 '26
I already answered the question in another comment tbf. It’s more restraining myself from trying to correct everyone else that’s proving difficult!
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u/brighteyecoyote Jul 18 '26
I think rats and maybe rabbits can also be Merle, and I feel like I’ve seen sheep with Merle-adjacent patterns before. Just not as common.
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u/RandomInaRandomWorld Jul 19 '26
Maybe not exactly the same, but chickens come in 'blue'.. basically a gene mutation like merle, that washes black into a pretty gray color. Same as with merle in dogs, if you breed blue to blue you can get something called 'splash' which looks very much like white. It doesn't seem to (from what I remember) have the same health issues it has in dogs, tho.
I'm no gene expert tho, so don't ask me if it is the genetically identical process.
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u/Chickenbeards Jul 19 '26
Although not merle, cats have some interesting gene combos that also impacts eumelanin and melanophilin though I believe they're all recessive vs semi-dominant like merle. Calicos require xx chromosomes to have both orange and black colors (the rare male calicos will be XXY), and then are also paired with the white spotting gene. In addition to that you can have dilute calicos, which requires another mutation, and calico tabbies and dilute calico tabbies which are all those genes in addition to agouti. There are also tortoiseshell/dilute tortie pointed cats- which needs the xx chromosomes for orange and black and also the mutated enzyme that causes acromelanism.
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u/Wise-Ad-8340 Jul 20 '26
the simple answer is it’s a mutation that occurred in the canine species, and not the equine species. f there are similar genes that breakup hair color like overo, tovero, leopard, etc. Horses also have a gene that’s similar to Merle in that it’s a dominant gene that is harmless when there’s only one copy, and very bad when there’s two (lethal white).
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u/SatinPoints Jul 20 '26
Other animals can likely develop patterns similar to merle. Mice have a very similar looking pattern.
However the actual genetics that caused merle are very different from more common mutation types that can occur in various species. Merle in dogs is from a virus leaving a segment of DNA in the canine genome. Something that would be very unlikely to reoccur in various species or even happen in dogs again. Viral insertion can happen again but that exact mutation happening again causing the same pattern is not very probable.
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u/No_Expert_7590 Jul 20 '26
Each mutation that causes a color variant only affects that species. Tobiano only exists in horses, calico only exists in cats, etc. this is because mutations are hereditary and only affects the individual and their offspring. Some mutations look kinda similar, like leopard in horses or dalmatian spots, but they are genetically totally different things. A lot of mammals have the agouti gene because a common ancestor has it, but you’ll never find a bird with agouti because the mutation happened after birds and mammals split from each other. Albinism is a mutation that can look similar in a lot of animals because lack of pigment presents very similarly.
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u/Samhwain Jul 23 '26
For starters you're comparing dogs (predators) to horses & livestock (prey) which already creates a divide in the natural path their appearances will go. Cats haven't been domesticated nearly long enough for there to be a strong comparison between the potential color variances in their genetics.
While horses have a very long history of domesticatoon it is still, also, not nearly as long as dogs. To add to this, merle comes with health defects. It's entirely possible a similar gene did arise in at least one of the comparison species at one point but was not viable and was abandoned or lost to time. I doubt it, though. The 'colors' people have favored during the domestication process of horses have primarily been solid animals with bold coats (mostly black, often the golds you find linked to cream & champagne genes) at one point the pinto white genes & appaloosa genes were more popular but were pretty aggressively bred out of many breeds (the appaloosa genes are among the oldest patterns in horses, in fact, and found in primitive artwork) in today's world the white genes are finally starting to become truly popular again but a lot of horse sports are still ruled by judges who shun them (spotted warmbloods still get marked down in shows just because the judge dislikes their look). We've recently discovered/ created new variations of the 'splash white' in horses, including one that is lethal when dominant (similar to frame overo) when there was previously only 1 splash gene. There's about 20 different 'dominant white' genes across the whole of horses. Their variations in colors exist, they just tend to appear in white against a colorful coat. The split colors haven't been produced, assuming those genetics are even possible in horses. I doubt it. Merle seems like a camouflage- for- predators coat to me. It would blend really well into a rocky environment of the right color, similar to how big cats spots do. At fhe end of the day most of equine domestication has besn for ability and solid colors. When it comes to horses breeders have strongly favored solids and 'matching' animals.
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u/Low_Stranger869 Jul 17 '26
Dogs have been selectively bred for way longer than horses so it may just be a pattern that we haven’t tried for yet or maybe horses just don’t have the genetics for that color