MEN, MICE and MERLE
by Linda McKay


 

 
This seems an unwieldy title, and I'd bet you are wondering what on earth these three all have in common, huh? Let's see if we can figure it out.

Some years ago P. J. Waardenburg, a Dutch eye doctor, discovered that people with two different colored eyes often had hearing problems. His documented research discovered that what is now called the Waardenburg Syndrome (WS) gene affects the body most often in three ways: hearing, pigmentation of the skin, hair and eyes, and facial structure. Some people with the gene are completely deaf (bilateral), others are deaf in one ear but hear normally in the other ear (unilateral). Another symptom of this genetic disease is two different colored eyes. Usually, one eye is brown and the other is blue but sometimes patches of brown and blue are mixed in the same eye. The blue eye color is often described as a "bright" pretty blue. Those with the WS gene may also have a distinctive coloring of the hair such as a white forelock, patch or streak starting at the front of the head. In addition to these hearing, eye color and hair color genetic expressions, cleft lip or cleft palate or a colon problem are fairly common in some affected with WS.
Now not everyone who carries the WS gene has hearing loss or two different colored eyes. Symptoms may be so mild that large families with many affected individuals may be unaware they are affected carriers. But the gene is dominant, and inheritance runs 50% regardless of the intensity of expression in carrier individuals.

There are mice called splotch mice because they have splotchy coloring in their coats. It was suspected the gene which causes this splotchy coloring in mice might be the same gene causing WS in humans. And sure enough, it IS the same gene which in humans causes WS. The gene is named pax3 and is on human chromosome 2.
By now you are putting two and two together and coming up with, I hope, four. Good, because there is more to come which may equal six. :-)

The cleft palate syndrome is a rare and isolated defect of (merle) in one family of Australian Shepherds 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. “ - C.A. Sharp, authority on Australian Shepherd genetics
Dogs of any breed with splotched coats courtesy of the merle gene may have eyes of different colors, one blue - one brown or eyes with different colors in them - patches of blue in a brown eye, for example. (Remember the WS symptoms in paragraph three?) There is now a DNA test for the merle gene, called the SILV gene in humans.

"Pigment-associated inherited deafness is not restricted to dogs. Similar defects have been reported for mice, mink, pigs, horses, cattle, cats, and humans. Deafness in blue-eyed white cats is common and is known to be passed on as an autosomal dominant defect. Blue eyes resulting from an absence of pigment in the iris, is common with pigment-associated deafness, but is not in and of itself an indication of deafness or the presence of a deafness gene.” - Dr. George Strain, Louisiana State University School of Veterinary Medicine

Merle Ocular Dysgenesis: Dogs affected with this condition will exhibit some combination of the following-- Microphthalmia (abnormally small eyes), acentric pupils, coloboma (a fissure in the iris) or other irregularities of the iris, lens luxation, cataract, retinal dysplasia or detachment, persistent pupillary membrane, equatorial staphyloma and lack of a tapetum (the reflective layer at the back of the eye). Since the condition is produced solely by breeding merle-to-merle, it can be avoided by not doing so.”
- 1998 Double Helix Network News, CA Sharp & Larry Green

It should be clear by now that blue eyes, deafness, visual problems, and the merle coat color are solidly linked in more than one species including man, and repeated merle to merle matings are like playing with matches. Sooner or later you will get burned. But what to do? Uninformed buyers see only the pretty colors produced by the merle gene, and the flashiest colored puppies in a litter are always the first to go, with solid colored pups going last if indeed they sell at all. Excessive white pups may be sound, but when bred it is my personal observation and that of many other breeders that they produce a higher percentage of excessive white progeny in succeeding generations than a line without an excessive white ancestor.

The answer to the merle dilemma is education, publicity, and stringent culling practices. We as breeders must get our solids competing and winning in the trials and conformation arenas and educate both ourselves and newcomers in our breed to the dangers of continual merle matings without at least an occasional merle to solid cross. In days past when Catahoulas had to work for a living, defective dogs eliminated themselves because they couldn’t see or hear well enough to stay alive. Now that the Catahoula is sold in ever increasing numbers to pet owners and show or hobby breeders, natural selection by survival of the fittest seldom occurs, therefore a breeder must cull his obvious homozygous merles (excessive whites) at birth if he wants to attempt to keep a reputation for producing sound “clean" dogs.

Very little research has been done on Catahoula genetics, so we must assume that work done with other breeds (and species) applies also to the Catahoula. There is much information available on new genetic research through libraries and the internet and even those who breed only an occasional litter have a responsibility to stay informed and as up to date as possible by utilizing these resources. This article only touches the surface of Catahoula genetic problems, but hopefully it will spark the readers’ interest in seeking out more definitive sources. But do beware of the many pet sites containing partial information on Catahoulas, and remember their game is to pretend to offer solid information on this breed while their real intent is just to sell puppies.

Many emails and letters were received after my article "Men, Mice and Merle" appeared in a Catahoula newsletter a few years ago, and after noting an increase in the number of excessive white dogs now being shown here is my attempt to address and clarify some of the questions posed:

What is excessive white? Any white outside the normal trim areas of blaze face, ring neck, chest, legs, and tail tip is considered excessive white, caused by the presence of two genes for merle (double merle). Although Catahoulas don't seem to suffer as seriously as other breeds from defects associated from being "double merles", there is no denying its presence, and the fact that lines using one or more excessive whites for breeding produce higher percentages of them. Another gene which produces white dogs and deafness is the piebald gene, which is also found in some lines of Catahoulas. Further information on the piebald pattern can be found here along with information on the harlequin gene which produces the so-called "patchwork" Catahoula pattern.

Many Catahoula females have very large litters, up to 14 or 15 pups. Many often have only 5 or 6. Something to consider when there is a small litter is the lethal white factor... perhaps there were originally 6 or 7 more pups in that litter of 4 or 5 born, but the others died early on because they were double merle, and the mother's body absorbed them. It happens.

Placement of white is generally what determines "excessive" white, and any merle dog with white outside normal trim areas (blaze face, ring neck, chest, legs, tail tip) must be considered defective until BAER (ears) & CERF (eyes) tested, because placement of white outside trim areas is the clue that the dog is a "double merle", ie: excessive white. Even if the head is fully colored, a dog with white bands, or streaks on other parts of its body is subject to the same unsoundnesses as a dog with an all white head and some of those defects may not show up until the dog is several years old. And guess what, folks... clear CERF and BAER tests in a puppy are no guarantee that eye disease or hearing problems won't develop later on in excessive white dogs.

If an excessive white is used for breeding, it should be BAER and CERF tested, its offspring should all be tested, and then their offspring tested at least for several generations. That is, if the breeder is concerned with keeping their lines sound. Some breeders are, some aren't, and some refuse to believe that excessive white is a problem... especially new breeders who have owned a couple of Catahoulas for a few years, then jump into breeding them and consider themselves instant experts after a few litters. If only it were that simple. Unfortunately, dealing with the consequences of the merle gene is anything but easy.

In my opinion, if a breeder can't afford to test every pup's vision & hearing, or doesn't want to test at all, they should not breed excessive whites. (But then I've invested several decades in breeding Catahoulas and tend to be pretty protective of this breed.)

There is another gene which is suspected to be present in our Catahoula breed, the piebald gene, which is a white base with large colored spots as found in the Walker foxhound and English Pointer to name two examples. At this point in time no one knows for sure if the piebald gene combined with the merle gene affects hearing more adversely than the double merle gene by itself. Many other breeds restrict the percentage of white allowed on their dogs because white is known to eventually "take over", and prevent display of any color at all on the animal's body.

This is a nutshell presentation on why buyers and new breeders should beware of using breeding stock with too much white. Respecting the evidence on record, an excessive white should be bred only if necessary in order to preserve extraordinary working ability, or a special bloodline or trait... in that case, breeding only solids to the excessive white dog will help overall soundness percentages, as will BAER and CERF testing all resulting offspring. Unfortunately, much incorrect or incomplete information about breeding merles & excessive whites has spread from new breeder to new breeder. This gives more new folks incorrect information on which to base their program, and they in turn spread bad information on to others. Proliferation of excessive white dogs through inept breedings hurts the breed in the present, and could destroy it in the future.

Everyone truly interested in preserving the Catahoula should be as protective of our breed's color as of the breed's working ability, and the only way to protect the integrity of the whole is to be ever watchful for excessive white as well as working traits. As the breed gains in popularity, new owners mostly want brightly colored dogs with blue eyes... new breeders don't realize and seldom want to deal with the fact that there should be a solid color infusion about every other generation to help keep that pretty color from degenerating into excessive white. Only by learning about merle genetics can we protect the Catahoula's beautiful color patterns for future generations. This isn’t just my opinion, it's in the research. With all the information available today through computers, everyone can easily do their own research on the merle gene via the Internet. Breeders of Shelties, Australian Shepherds, and Great Danes have done the bulk of the work and it's available to anyone who's willing to spend time looking for it. Here's a good place to start... Elementary Merle Genetics.

So here's my vote for showing and campaigning and promoting and breeding more of our solids! It would help the breed greatly if all registries which support conformation shows for Catahoulas would write into their standards that dogs should not be judged on eye or coat color, or at least instruct judges not to mark a dog higher or lower because of its splashy color or lack thereof.