The Trouble With Merle
     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 bitch 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 she know this might happen ahead of time? How about the stud owner? If either or both of them knew, how could they allow it 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-merle Aussies 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 Specialty 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 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 gene that produces it can also cause serious problems for a dog that inherits two copies. Homozygous merles—those with two merle genes—almost always have defects in sight and hearing. This is the reason that all Aussie breed standards used since the early 1970s have discouraged or disqualified “excessive white.”

The definition of “excessive” is sometimes hotly debated.  In the past, fans of flashy trim tried successfully to adjust the breed standard to allow a little more, specifically on the ears. Others damn it as a blight on the breed.

The standards have discriminated against extreme amounts of white because this type of coloration is usually—though not always—associated with merle homozygosity. An animal can also have too much white because of genes at the “S” locus, which codes for greater or lesser white markings. [See “Color Clashes”] Since homozygous merles—also called double merles, white merles, excessive whites or lethal whites—may have defects and since most homozygous merle Aussies are predominantly white, the framers of the breed standards discriminated against the color in order to discourage people from breeding 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 decide before you breed just what you will do in the very likely case that you get homozygous merle 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 upbeat, outgoing temperament. But a vet I once worked for had so many negative experiences with them that she felt poor temperament was characteristic.  I have never seen anything that links temperament to color, but the sensory deprivation caused by the merle defects probably exacerbates any inherent temperament weakness the dog may have.  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.

Deafness in homozygous merles results from a lack of pigment in the inner ear.  Hearing loss can vary but many of these dogs are profoundly deaf. Lack of pigment (white hair, pink skin) is associated with deafness in a number of species of mammal, including man. Dogs that have white markings on or around the ears due to white trim genes may experience the same kind of deafness.

The eye defects have been well described in the veterinary literature both here and abroad. They are extremely variable. Some individuals have only minor vision loss, but most are more seriously affected. Merle homozygotes will have some combination of defective irises, persistent papillary membrane, cataracts, subluxated lenses, and/or retinal defects. Interestingly, the amount of white even 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 breeder calls me 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 reply.

“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 and white puppies in the litter?”

“Oh.”

I’ve had several conversations like this over the years.  People in Aussies refer to these dogs as “phantom merles.” The more correct term is a cryptic merle. Since it is possible for a merle to have only a tiny amount of blue, it is also possible, though highly unlikely, that a dog might have a merle gene but exhibit no merle spots at all. There is no way a breeder would know this had happened.

When a non-merle, bred to a merle, produces pups with excessive white, many people are suspicious that the non-merle might be cryptic.  More likely, it and probably the mate carry genes for too much white trim. But if eye problems typical of a homozygous merle are found in the excess white pups, the apparently non-merle parent could actually be a merle. An ophthalmologist experienced in viewing merles should be able to tell the difference between merle eye problems and other diseases. If you suspect a dog is a cryptic merle, breeding it to another non-merle will tell you.  If you get at least 6 pups and none of them are merle you have a 98%+ probability that the dog is not merle. If you get even one merle pup, it is and any future breedings should be done with its true, though hidden, color in mind.

Our fondness for merle is yet another reason why we should have our dogs eyes checked by an ophthalmologist. Sometimes a homozygous merle will have “normal” merle coloring. A friend of mine had one. “ET” had only a moderate amount of white trim: Stripe, chest, one leg and her feet. She was from the Woods line, known for little or no white and very deep pigmentation. ET was medium blue and therefore “light” in color for her breeding. . She also had eye defects typical of a homozygous merle.

Variation in merle pigmentation is extreme, going from the virtual solids cryptic merles to a mostly light-colored animal with a peppering of full pigment. The color of the “blue” areas may vary from pale silver to deep gunmetal blue. Red merles may be anywhere from a buff to the color of iron-rich earth. The dark areas should be liver but appear more orange on some dogs.

Areas of intermediate pigment, called dilution spots, occur on some dogs and are probably caused by modifying genes. Animals with dilution spots are more likely to throw puppies that have them than those which do not.

There is a gene that modifies the merle pattern called “tweed.”   It was identified and was first described in the Australian Shepherd. Tweed merle dogs have 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.

Tweed gene is a dominant that expresses itself only in the presence of merle. Therefore, solid color dogs are not affected even if they have the gene. Interestingly, homozygous merles also do not exhibit tweed. 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 pass on, so its offspring from a non-merle should always be merle and its offspring from a heterozygote will be either “normal” merles or homozygotes.

However, there is documentation of homozygous merles producing non-merle offspring. My bitch was one of them. Homozygous Aussies that produced non-merle offspring were the subject of a scientific journal article. This has also been reported in Collies and Shetland Sheepdogs and indicates that there is something inherently unstable about the merle gene.

Like a sharp knife, merle has many 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. 

Copyright © 1996-2002 by the author. No reproduction of any kind without the author's express permission. C.A. Sharp is a Member of the ASCA DNA Committee.