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The Problem With Merle Coat Patterns in Dogs

  1. #1
    Join Date
    Mar 2014
    New Hampshire

    The Problem With Merle Coat Patterns in Dogs

    Merle ó that kaleidoscope of swirly patterns that has no two dogs looking alike. Itís one of the most beautiful coat patterns in the dog world. But merle is definitely a case where too much of a good thing is, well, a bad thing.
    The merle (also called dapple) pattern is the pattern in which random splotches of dark pigment are overlaid over a lighter shade of the same color. Itís commonly seen in Catahoula Leopard Dogs, Australian Shepherds, Collies, Shetland Sheepdogs, Dachshunds, Cardigan Welsh Corgis, Great Danes, and less commonly in many other breeds such as Chihuahuas, Border Collies, Pyrenean Shepherds, Beaucerons,Pomeranians and Cocker Spaniels.

    Why you shouldnít breed two merle dogs together

    Merles are popular, so it seems only logical to breed two merles together to get more merles. No. Donít do it.
    The merle pattern is produced when a dog has a single copy of the M< allele. All merle dogs have the genotype Mm ó meaning they have one allele for merle and one allele for non-merle. All non-merles are mm. If you breed a merle (Mm) to a non-merle (mm) you will on average produce a litter in which a half of the puppies get the M allele so are Mm (merle) and half get the non-merle allele so are mm.

    But if you breed two merles together (Mm X Mm) you will produce on average a quarter mm (non-merle), a half Mm (merle) and a quarter MM (double-merle; also called double-dapple). And double merles donít look like merles. Instead, theyíre mostly white with merle patches. But the main reason you want to avoid producing MM dogs is that they often have visual and auditory problems.

    What are the specific health concerns for merles?

    If you like tech-talk and numbers, read this; otherwise, skip ahead: In a study of several merle breeds, merles with one copy of the M allele had a rate of 2.7 percent deaf in one ear and 0.9 percent deaf in both ears; double-merles had a rate of 10 percent deaf in one ear and 15 percent deaf in both ears. Interestingly, the rate in merle Catahoulas (5.9 percent) was lower than that in other breeds (for example, 9.4 percent in merle Australian Shepherds), and especially lower in double-merle Catahoulas (10.3 percent) compared to other double-merles (55.7 percent in Aussies and 85.6 percent in all other breeds combined). The lower incidence in Catahoulas may reflect the smaller amount of white Catahoula double-merles tend to have. Again, nobody knows why. Blue-eyed merles have no higher incidence of deafness than brown-eyed merles.

    Just because a dog is double-merle, donít assume heís deaf. Dr. George Strain of Louisiana State University is the go-to expert on coat color and deafness in dogs. One of the other coat patterns heís studied is the piebald gene, which can create mostly white dogs like Dalmatians. He says the prevalence of deafness in dogs is higher in double merles than in single merles, but the relative risk of deafness was less than that in Dalmatians and white Bull Terriers (although greater than that in other dog breeds with the recessive piebald alleles).

    Double-merle dogs often have an additional problem, microphthalmia, in which the eyes are abnormally small (sometimes barely there) and often nonfunctional. As of yet, the way in which the merle gene affects this is unknown. It does not appear be through an association with the gene known as MITF (microphthalmia transcription factor), however.
    Aside from these auditory and visual problems, double-merles are otherwise healthy. And not all double-merles have even these problems. Some are absolutely fine. But why take chances? Never breed two merles.

    But hereís where breeding can be tricky. Many breeds with merle also have other genes (at the s locus) that cause white on dogs, and this white isnít associated with problems caused by being a double-merle. For example, many Collies have white feet, ruff, blaze and tail tip ó but this is caused by the s allele, not MM. And some Collies can be mostly white, but again, this pattern can be caused by a different s allele. When these dogs are white with sable (Lassie-colored tan) itís easy not to confuse them with double merles. But what if they are white due to the s allele but combined with Mm? The dog would be white with merle, and could be confused with a double-merle. This is why itís essential to know a dogís parents before jumping to conclusions!
    And it gets even trickier! Sometimes merle dogs have so little merle you can hardly tell them apart from non-merles ó but theyíre still genetically Mm. If you breed one of these ďcrypticĒ merles erroneously assuming heís mm you could produce double merles. If you find even one tiny spot of merle in your dogís coat, assume he is a merle. Not sure? Thereís a DNA test available that will let you know if your dog has the M or m alleles.
    Youíll sometimes read that breeding merle to merle ďis only for experienced breeders.Ē All the experience in the world wonít change how the genes segregate and how they influence a dogís health. What they mean is not to do it unless youíre prepared to deal with deaf or blind puppies. And while such dogs can make wonderful companions, those with normal hearing and vision do have an easier time in life
    he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion

  2. #2
    Join Date
    Aug 2017

    Now I have learnt something new

    Though I have read so much about dogs, I wasn't aware that the two coat patterned merle dogs shouldn't be breed together. Would love to know more about such cautions to secure dog health

  3. #3
    Join Date
    Mar 2014
    New Hampshire
    If you want more tips on pretty much anything read through some of my former posts
    he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion

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