Color Blind?You may have heard of "lethal whites" or homozygous merles. To accurately cover the subject of homozygous whites, you'll need to know a little bit about the merling gene. Aussies come in two acceptable types, solid (m) and merle (M). The m's represent this. The merle gene is dominant and this is represented by its capital. Therefore, solid is recessive. An Aussie has a combination of these two genes. The combination may be mm, Mm, or MM. mm is a solid dog (such as black tri). Mm is a heterozygous (meaning mixed) merle (such as blue merle). MM is the homozygous (meaning like) merle, this is the "Lethal White" Aussie.
Lethal whites (known hereafter as homozygous because lethal white is a misnomer) are a result of a merle to merle breeding, but not every puppy in the litter is a homozygous white. One out of every four (in odds) will be a homozygous white.
A lot of people e-mail me to ask if they should be adopting a possible homozygous merle, concerned with health. Any negative aspects of this syndrome are
congenital. That is, they are evident at birth. If a dog appears normal at
six months, its condition should not worsen.
For more information, there is a great website to check out, The White Aussie Project that should be far more indepth than what you have seen here.
C.A. Sharp, authority on Australian Shepherd genetics:
Predominantly white Aussies are almost always the result of merle-to-merle breeding. These homozygous (meaning two like genes) merles, have inherited the merle color gene from both parents. They are usually, but not always, blind and/or deaf. The defects can be quite variable, so the amount of vision and hearing loss will range from little to total--with most being toward the bad end of that scale.
If you breed a homozygous merle Aussie (or Sheltie or Collie or BC)--and I *DO NOT* recommend this to anyone without considerable experience in Aussies or other merled breeds--you should select a non-merle mate. All the resulting offspring will probably be merle. I say probably, because sometimes something called "germinal reversion" occurs, and a homozygous merle will produce a non-merle pup. This has been reported in the scientific literature in Aussies and there are anecdotal reports of it happening in Shelties and Collies. These non-merle pups are not "phantom merles (solid-appearing dogs with Merle genes)." If bred to other non-merles, all their pups are non-merle.
Homozygous merle Aussies are usually, but not always, predominantly white. This is why the standards discriminate against white coloration and why people are discouraged from keeping white pups. But it is possible to have a "normally" marked merle that is homozygous.
Betty Nelson (long the chair of ASCA's former Genetics Committee) had a homozygous merle bitch who was a medium blue with a stripe down her nose, a white throat and chest, one white foreleg and white toes. Hardly "full white trim." But for her line (Woods) this was a LOT of white, since they generally have almost none, and she was light in color compared to the the normally deep pigmentation of that line.
I once asked a Doxy breeder about merles in his breed. (They call them dapples.) They are also a breed that does not allow white markings. He told me some lines had homozygous merles with a lot of white and some had very little.
There are also predominantly white Aussies which are not homozygous merles. This results from the action of genes at the S-locus--the same one that gives us our normal white markings. There are several alleles (forms of the gene) at this locus, for everything from no white (as in a Labrador) to almost all white (like some Fox Terriers). White Aussies that were not homozygous merles used to be much more common than they are now. I can remember seeing several when I got started back in the early 70s. Look at the historical section of ASCA's first yearbook for pictures of a few more--Harper's Old Smokey (whose name you should all recognize) is one of them.
They are much more rare today, because we have been slowly eliminating the "more white" form of the S gene from our gene pool by culling or at least not breeding extremely white Aussies.
If you don't want to produce homozygous merles, it's real simple--never breed merle to merle. That's the route I took when I was breeding. (Moby, my white bitch, was donated by another breeder.) If you do plan merle-to-merle breedings, you better decide before hand what you plan to do with the homozygous pups, because you will get them.
If you want to keep a homozygous merle, you had better be willing to devote yourself to keeping it safe from dangers it cannot hear or see for its entire lifetime.
Moby was one of the sweetest Aussies I've owned, but I've known of other homozygous merles whose temperaments were terrible. It probably depends on the strength of the individual dog's genetic background, but the sensory deprivation and tendency to be startled by the unexpected because the dog couldn't see or hear it coming may push borderline dogs over the edge.
If you choose to breed a homozygous merle because it has traits unrelated to its color that you want (and unless you have a LOT of experience with this breed, like Shiela does, I don't recommend it) you need to be aware that the merle gene is not the most stable bit of DNA in the genome.
Not only can you sometimes get non-merle pups (which should not happen according to the rules of Mendelian genetics,) worse things can crop up, too. The "cleft palate syndrome" is a rare and isolated Aussie defect that has been the subject of numerous scientific journal articles and symposia presentations. It is a sex-linked (on the X chromosome) defect, in which females have minor abnormalities, like extra toes, while males die of massive skeletal abnormalities and a cleft palate. This disease STARTED with a homozygous merle bitch that just happened to be born, and kept for coat color research, when ASCA's genetics committee got started. The odds of a random mutation are high enough by themselves (nothing like this has been seen anywhere in the breed but in the descendants of this bitch.) The odds of it happening in the very bitch who was selected to kick-off color research in the breed, is even higher.
I feel it is no coincidence that the breeds in which merle is most common are also breeds heavily plagued with eye defects. The defects in CEA mimic some of those in merle occular dysgenesis (homozygous merle eye). PPM, iris coloboma and cataracts also occur in homozygous merles.
Homozygous merles will also sometimes have other physical defects, but these are better documented in Collies and Shelties than Aussies. I don't know if this is because we just don't have many or because we keep very few of these dogs up and therefore don't have enough data.
If you do keep a white dog, you should be doing everything you can to educate people who see that dog and might think it's pretty about what is wrong with homozygous merles. ARPH has enough trouble placing sound dogs, they don't need more pound puppies who were dumped because ignorant puppy-buyers discovered after the fact that white Aussies have problems or decided that their originally noble intentions were becoming too much bother.
If you produce a homozygous merle, you have a responsibility to take care of it one way or another. If you acquired it, you have made a life-long commitment to caring for a handicapped animal. If you want to breed it, you better be careful and know what you are doing.
For more, click here to find out more about Deafness in Whites