The Trouble With Merle First published in Double Helix Network News, Vol. IV No. 2, Spring 1996 by C.A. Sharp A breeder waits anxiously by the whelping box as her prize bitch breaks off her panting. The bitch’s mottled black and silver flanks, distended by late pregnancy, tense with another contraction. A membrane-wrapped bundle slides out of the birth canal. The bitc h goes to work, cleaning her newborn. As the sack comes off, the breeder winces. Several hours later the bitch sleeps on her side. Nine squirming puppies nuzzle her belly. The breeder sighs. Three of the puppies are mostly white. Each of them has inherited genes—one from each parent—which cause major defects. Every breeder’s nightmare? Did the breeder know about this ahead of time? How about the stud owner? If either or both of them knew, how could they allow this to happen? Some European breeders once made a serious effort to ban exhibition of dogs carrying this defective gene. It is called merle. Merle is the Australian Shepherd’s signature color. So much so that the uninformed often mistake non-merles for some other breed. I remember a prominent breeder of years gone by telling me of an animated discussion she had with a self-appointed expert she met in a local park where she was practicing obedience with her National Specialy winner. The man insisted her dog was a Border Collie. Nothing she could say would convince him. Finally she agreed that he must be right, just so she could get on with her training session. Many Aussie owners have had similar experiences. There is only one other breed—the Catahoula Leopard Dog—where merle is more common than in the Aussie. We like the color. It is distinctive and beautiful. Unfortunately, the genet that produces this beautiful color is a semi-lethal. Homozygous merles—those with two merle genes—almost always have defects in sight and hearing. This This is the reason that all Aussie breed standards used in the past quarter century have discouraged or disqualified “excessive white.” Just what is “excessive” is hotly debated by extremists on either side of the fence. Fans of flashy trim try every so often to adjust the breed standard to allow a little more. Purists whose lines have little or no white damn it as a blight on the whole breed. Both positions are ill-informed. The standards have discriminated against extreme amounts of white because this type of coloration is often—though not always—associated with merle homozygosity. (An animal can have “too much” white because of genes at the “S” locus, which codes for greater or lesser white markings. [See “Color Clashes,” DHNN, Winter 1996.] Since homozygous merles—also called double merles, white merles, excessive whites or lethal whites—may have severe defects and since most homozygous merle Aussies are predominantly white, the framers of the breed standards discriminated agains the color in order to discourage people from keeping these dogs. There is a very simple way to avoid producing homozygous merles. Never breed two merles together. This is the option many breeders choose. However, sometimes the stud most ideal for your merle bitch is also a merle. If that is the case, you must be willing to cull puppies. While a blind, deaf dog can live a full happy life—I know because I had one—it is not always the case. My dog flourished because of her up-beat, outgoing temperament. A fet I once worked for had so many negative experiences with them that she felt poor temperament was characteristic. The sensory deprivation caused by the defects probably exacerbates existing temperament problems. Whatever its temperament, a blind and/or deaf dog is a responsibility no ethical breeder should knowingly foist off on anybody, no matter how well-meaning they might be. The ear defects in homozygous merles have not been well studied, but lack of pigment (white hair, pink skin) is associated with deafness in a number of species of mammal, including man. The eye defects have been well described in the veterinary literature both here and abroad. They are extremely variable with some individuals having only minor hearing or vision loss, but most are more seriously affected. Interestingly, the amount of white in homozygous merles does not correlate to severity of defects. In a study of the embryonic origin of merle eye defects, Dr. Cynthia Cook, of the University of California, San Francisco, observed that severity of defect and amount of pigment were not related. Common knowledge tells us that to avoid merle problems, never breed two merles together. The problem with common knowledge is that it may get the big picture but it overlooks the details. Consider the following scenario. A breeder4 calls em with a problem. A puppy in his new litter is solid, except for this little bitty merle spot on its tail. “What color is it?” he asks. “Merle,” I replay. “But I’m gonna dock the tail!” “Sorry, it’s still merle.” “I could just register it as black.” “OK,” I say. “Then what happens when someone breeds this ‘black’ pup of yours to a merle and, whoops, there are eye defects in the litter?” “Oh.” This particular exchange is fiction, but I’ve had similar conversations several times. I noted a recent ad in on of the breed magazines where an apparently solid color dog was advertised as a “phantom merle.” I don’t know about that particular animal, but I suspect he either had a now-absent blue spot on h is tail or a tiny one elsewhere that isn’t apparent. One of our breed’s major foundation sires, Fieldmaster of Flintridge, was such a dog. He had a small patch of blue on his neck but otherwise appeared to be a black tri. He had papers with two registries. One of them described him as black, the other as blue merle. Since it is possible for a merle to have only a tiny amount of blue, it is also possible—though unlikely—that a dog might be genotypically merle and phenotypically non-merle. There is no way a breeder would know this had happened. If a non-merle, when bred to a merle, produces pups with excessive white, you might be suspicious. If eye problems crop up the situation gets muddier. The eye defects resulting from merle eye anomaly are almost always multiple and are also distinctive. An ophthalmologist experienced in viewing merles should be able to tell the difference between merle ocular dysgenesis and , say, Collie Eye Anomaly. It is important that you seek an ophthalmologist with such experience because sometimes a homozygous merle will have “normal” merle coloring. One normal-looking homozygous merle I knew had only a moderate amount of white trim: Stripe, chest, one leg and the theree other feet. She was from the Woods line, known for little or no white and very deep pigmentation. This bitch was medium blue. She had several litters and never produced a non-merle pup. Bred to another merle, she produced ups that were clearly homozygotes. She also had eye defects typical of a homozygous merle. Variation in merle pigmentation is extreme, going from the virtual solids described earlier to a mostly light-colored animal with a peppering of full pigment. The color of the “blue” areas can also vary from pale silver to deep gunmetal blue. Red merles may be anywhere from a buff to the color of iron-rich earth, though the liver areas will also generally vary. Areas of intermediate pigment, called dilution spots, can also occur. This variation is probably caused by modifying genes. Animals with dilution spots are more likely to throw puppies that have them than those which do not. One of these genes, “tweed,” has been identified and was first described on the Australian Shepherd. It causes extremely varied merle pigmentation. A blue tweed dog will have black spots, charcoal spots, slate blue spots, light blue spots and even dark brown spots. Red tweeds also exist. The tweed patterns are often remarkably regular and the result can be stunning. However there is a hitch. It can also cause white spots in places you don’t want them. The tweed gene is a dominant which expresses itself only in the presence of a merle gene. Therefore, solid color dogs are not affected even if they have the gene. Interestingly, homozygous merles also do not exhibit tweed patterning. The reason for this is unknown. Merle is not a gene that behaves itself. A classic incomplete dominant gene will exhibit three distinct types—one each for the homozygous dominant, the heterozygote and the homozygous recessive. If a homozygous merle is bred, it has nothing but merle genes to p ass on, so its offspring from a non-merle will always be merle and its offspring from a heterozygote will be either “normal” merles or heterozygotes. However, there is documentation of homozygous merles producing non-merle offspring. My bitch was one of them. This has also been reported in Collies and Shetland Sheepdogs and indicates that ther is something inherently unstable about merle. The fact that hereditary eye defects are common in Aussies and Shelties and epidemic in Collies may be more than an interesting coincidence. A German study of dapple (merle) Dachshunds found eye defects in most homozygotes and a significant number of heterozygotes. In non-merles, such defects were very rare. While this doesn’t exactly equate to the situation in Aussies, Collies and Shelties (non-merles are also affected,) it is a point to ponder. True coincidences are rare. Another involving merle is the following case: The Australian Shepherd Club of American became interested in studying odd colors and the problems associated with merle in the mid-seventies. A Color Genetics Committee was formed. Betty Nelson, its chairman, happened to have a merle-to-merle litter and kept a homozygous pup for the committee. That bitch pup carried a mutation which had not been reported previously. She is the founder of the line of research dog carrying the cleft palate syndrome that was the subject of numerous veterinary and human medical journal articles and symposia presentations. The defect was a sex-linked lethal. Sex-linked defects are carried on the female X chromosome. Carrier bitches would have extra toes and mild distortions of their limbs. Males which inherited the defect-carrying X chromosome from their dams would have severe skeletal deformities and cleft palates. All but one of these affected males died in infancy. The cleft palate syndrome is of no concern to Aussie breeders since it occurred only in this resea4rch dog’s descendants. However, the possible instability it suggests for he merle gene should be. Ann Bowling of UC Davis stated in one of the early papers on the syndrome that she doubted the mutation occurring in a merle homozygote was a fluke. Like a sharp knife, merle has a variety of virtues but it can also damage. Breeders of merle dogs should be aware of its negative potential. Breeders of Australian Shepherds, the majority of which are merle, must be doubly so.