Pigment and Health
Merle and White Spotting Genes
This article was originally written in 2005, just prior to the identification of the merle gene and the subsequent development of the DNA test for merle, and the 2007 discovery of the MITF as the gene responsible for "S" white spotting. It has been updated to reflect these discoveries.
The Relationship of Pigment to Health
by Geneva Coats, R.N.
Rare and exotic colors...one of the most attractive and exciting features of our Pomeranians. Variety is the spice of life! Different styles appeal to different breeders. However, there are some important pigment-related health issues to consider.
Since the inception of purebred dog registries, standards have been established for all breeds. The standard serves to protect a breed from fashion whims. It strives to maintain a breed's traditional "look" and purpose. The standard establishes desirable conformation goals, temperament characteristics, and suitability for the original purpose of the breed. The standard provides a goal for breeding practices. This is why standards are written in the first place, to act as guides for breeders and judges. You don't need to understand all the details and theories of genetics, but you do need to abide by the standard when breeding and judging.
Our Pomeranian standard was changed in 1997 to allow all colors, patterns, and variations thereof. This may serve to promote some breeding practices which might ultimately be detrimental to the health of the breed. We need to examine this issue carefully. The direction we take in breeding is the path to our future. The desire for health and concern for future generations must always take precedence over the desire for aesthetics.
Why is a color or pattern so important to the stewards of a breed? Breeders have always been keen scientific observers, even before genetics became an established field. They have worked tirelessly toward a goal of producing healthy dogs. Dog with health issues are in limited demand as companions, and often unsuitable for work. Some breed standards disallow excessive white, dilute colors or merle. Many breed standards specify that parti-color dogs have a certain proportion of color to white, or that they must have color extending over both eyes and ears. Most all standards call for full, dark pigmentation of nose, lips, eye rims and pads. Albinism is generally disallowed. These guidelines were not established for aesthetics' sake, but because certain colors, patterns, and lack of pigmentation were known to be associated with an increase in certain specific health problems. There is much documentation and information related to the merle and white spotting patterns, and they will be the focus of this article.
COAT COLOR DEVELOPMENT
The process of coloration and color patterns in dogs starts with embryonic development. The cells which become melanocytes (pigment producing cells) are derived entirely from the neural crest of the embryo. Pigment cells develop from the same cells that give rise to the nervous system. Defects in certain color genes can be associated with defects in the nervous system, as evidenced by vision and hearing problems. Color provides a logical genetic indicator, and explains why it is likely that certain patterned dogs, such as extreme piebalds (>90% white) and merles may be at risk for specific health problems. In studies to date, the coat patterns resulting from genes producing white are significantly associated with deafness. In addition, suppression by "white genes" of pigmentation in the iris of the eye is also associated with deafness. On the other hand, the presence of pigmented, colored patches in the coat is associated with reduced risk of deafness.
WHAT IS MERLE?
Merle is regarded as a coat pattern, not a color. The merle gene works like other dilution genes, in that it lightens whatever color is already there. Whatever the basic color of the dog-unless he is all white-the merle gene will cause dilution and splotching imposed on the base coat color. The appearance is a speckling or marbling, like when bleach is splashed on your clothes. The pigment cells are affected individually in the fetus. Some are disabled completely, leaving white areas. Some cells allow partial expression of pigment, leaving merled or dappled areas. Some cells are unaffected, leaving the coat color normal or non-merled. The amount of dappling or merling can vary. The dog can have perhaps just a small dappled spot somewhere, or perhaps just a light eye with a nondappled, normal coat. Or, it might display dappling throughout the coat. The effect is most noticeable on a black coat; where the black is diluted to grey, the color is called "blue merle".
Merle appears at first glance to be dominant, because if a dog carries one gene for the merle factor, he will be affected by it. However, merle is different, because it is incompletely dominant. A single merle gene, inherited from one parent, produces what is known as a "heterozygous" or "single" merle. This single copy of the gene will usually cause the affected dog to display merling somewhere in the coat. This can range from light dappling to extensive dappling. Sometimes the effect on the coat is not evident, and there is no way to tell at maturity that the dog carries the merle gene. Sometimes, blue flecks in the eyes are the the only sign that a dog carries the merle gene.
If two heterozygous merles are bred together, statistically 25% of the offspring will inherit two merle factor genes, one from each parent. When this happens, we produce what is known as a "Double (homozygous) merle". The effect of the gene is then doubly intense. There is so much color dilution that the dog is usually predominantly white, and almost always has impaired hearing or vision-or both.
In Great Danes, merle interacts with another gene called harlequin, which bleaches the gray areas of the coat to white. In the pure homozygous form, harlequin is lethal. In Danes and Aussies, the tweed pattern is a variation on merle which adds a third, intermediate shade to the coat.
Many double merle dogs are so defective that they do not survive to birth. (estimates are as high as 50% mortality in utero). Those that survive are the ones with "just minor" defects...minor enough to handicap rather than kill. A gene that destroys half of those carrying it in a double dose in utero cannot be dismissed or ignored.
In French, the word "merle" means "blackbird", and "le merle blanc" ("The white blackbird") is an expression, meaning something that is impossible or something that cannot exist. This is an admirable goal for the white merle dog!
HOW MERLE WORKS
Merle is a gene that is inherently unstable. It is a "transposon", or transposable element; a mobile parasite segment of DNA in the chromosome. Transposons usually originate from viruses. A virus is a particle containing DNA which inserts itself into the host cell, thereby infecting it. If the virus does not kill the host cell, it can leave behind a piece of its genetic material in the DNA of the host cell. When this happens in the sex or germinal cells, the particle can become a part of the genetic makeup of some of the offspring. This is a way that mutation occurs.
Transposons have also been called "junk DNA or "selfish DNA", as they rarely provide any benefit to the host cell. Transposons are also often referred to as "jumping genes." The transposon can move to different positions in the cell in a "cut and paste" process. In so doing, it may cause mutations, or alter the amount of DNA in the genome. The merle transposon segment may lose some of its DNA particles during cellular replication during early embryonic development. Coat color in that area will then be less affected by the gene, or even unaffected, and the underlying true coat color will be expressed. This explains why there are some areas of the coat which are NOT dappled, and other areas which ARE.
The decorative variegated Indian corn is produced by transposon elements in the corn gene. Many viruses, such as those responsible for AIDS and feline leukemia, function as transposons. In humans, hereditary diseases such as hemophilia A and B, predisposition to colon polyps and cancer, and Duchenne muscular dystrophy, are all related to inherited mutations produced through the transposon process. Genetic engineers sometimes use transposons to try to intentionally introduce certain genes into an organism.
There are also documented cases of homozygous merles producing non-merle offspring ("germinal reversion"). This is further evidence of the instability of the gene. Not only can you sometimes get non-merle pups (which should not happen according to the rules of Mendelian genetics), but worse things can happen. 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 was kept for coat color research.
PIGMENT PLAYS A ROLE IN HEALTH
Many breeds have the white spotting and merle factors in their gene pools. These genes are not colors per se, but do affect the expression of coat color. In breeds which have traditionally included the merle color pattern, the predominantly white (double-factored) merle has been disallowed in the show ring, as well as selected against in the whelping box. And with very good reason. The gene which produces the merle pattern is associated with some very serious health defects. Both the white spotting gene and the merle gene are known to sometimes affect hearing. The merle gene, however, is unique in that it interacts in an as-yet undetermined manner with genes for eye features.
MERLE AND EYESIGHT
There is a complex of eye defects associated with the merle gene. These defects can be superficial in nature, such as a difference in color between the iris of one eye and the other. The example of this is a dog with one brown eye and one blue eye. (Note that this trait is not necessarily indicative of possession of the merle gene, because it can also be found in dogs with extreme white spotting). In addition to superficial indicators there are also major effects, such as absence of the reflective substance which lines the back of the eye, resulting in reduced ability to see in low light. There can be lack of retinal pigment, which directly reduces vision. Small or absent eyeballs, irregular or starburst pupils, and clefts in the iris can occur, as well as persistent pupillary membrane, strabismus (cross-eyed), lens luxation , and juvenile cataracts.
THE MERLE LINK TO MICROPHTHALMIA
The term "microphthalmia" means a smaller than normal, tiny eyeball due to a defect in early development. Microphthalmia is a defect very commonly associated with merle; particularly homozygous "double" merle, it even rarely occurs in heterozygous or "single" merles. Complete absence of the eyeball in the eye socket sometimes occurs. This is known as anophthalmia.
MITF, or "Microphthalmia Transcription Factor", was discovered in 2007 to be the location of the gene that affects pigmentation, causing white spotting (otherwise known as the particolor or piebald pattern).
The merle gene is believed to affect eye development through some sort of interaction with MITF. Now, why would microphthalmia occur in association with merle and not just with white coat color in particolor dogs? I've asked several canine geneticists, but have received no answer! This relationship is still a mystery. The exact mechanism by which merle interacts with the white spotting gene and produces the defect of microphthalmia is still waiting to be discovered!
MERLE AND HEARING
Hearing impairment can occur in merles due to a lack of melanin or pigment in the inner ear. Not all merles will develop problems with hearing, it all depends on whether or not the inner ear is pigmented. If the inner ear isn't pigmented, the nerve cells responsible for hearing can't develop as they should. Then, the nerve endings atrophy and die off in the first few weeks of the puppy's life, resulting in partial or total deafness. The deafness is neither dominant nor recessive, but is linked to the merle gene which disrupts pigmentation and secondarily produces deaf dogs.
White outer ear color is often associated with lack of inner ear pigment and deafness. However, sometimes deafness occurs even with patches of color on the ears. Mild to moderate hearing impairment may never be noticed, and is rarely tested for.
ONE COPY OF THE GENE HARMLESS?
The popular belief is that heterozygous merles are completely normal, and that only homozygous merles have health defects. Heterozygous merles can have expression of lesser defects of the eye such as clefts in the iris, and a thinning of the retina (similar to what is found with Collie eye anomaly). There are also rare cases of microphthalmia in "single" merles. Deafness can also sometimes occur, if the merle gene prevents pigment deposition in the nerve cells of the inner ear during embryonic development.
Heterozygous "single" merles often have reduced eye pigment which produces the characteristic blue eyes. Interestingly, the amount of white even in homozygous merles does not correlate to severity of eye defects. In a study of the embryonic origin of merle eye defects, Dr. Cynthia Cook, of the University of California, San Francisco, observed that the severity of eye defect and amount of pigment were not related. This is in contrast to hearing impairment in merles; the likelihood of deafness increases with increasing amounts of white in the hair coat.
The Hannover Veterinary School in Germany conducted studies on Dapple (merle) Dachshunds. Their studies demonstrated eye problems, sperm imperfections, and hearing impairment. These problems were found in homozygous merles, and also heterozygous merles. Hearing faults, ranging from slight hardness of hearing to complete deafness, occurred in 54.6% of homozygous merles and 36.8% of heterozygous merles. As a result of these studies, in 1986 it was suggested that FCI restrict the breeding of merles on welfare grounds. These studies, however, are now generally considered outdated. Future testing of vision and hearing in merle dogs will hopefully paint a clearer picture for us.
GENETIC LINKAGE CAN CAUSE SPREAD OF DEFECTS
Genes on the same chromosome are usually inherited together. However, during cellular division and replication, sometimes chromosomes will randomly break and recombine. Genes that were formerly associated with each other can then become separated and inherited independently. This process is known as genetic linkage. We have already mentioned the Aussie cleft palate syndrome which originated with a merle bitch. Iris coloboma, or missing part of the iris, is being reported in non-merle Aussies. Coloboma appears as a notch at the edge of the pupil, giving the pupil an irregular shape. This sight-altering defect is believed to have been brought into the breed through linkage with the merle factor. This trait is now inherited independently from merle.
It is likely more than coincidence that breeds in which merle is most common are also breeds which are heavily plagued with a variety of eye defects. Yearly CERF testing by a veterinary ophthalmologist can help screen out heritable abnormalities. Further studies in merle dogs are needed to observe the frequency and exact methods by which eye defects are produced. It is theorized that some mutations are produced by damage done to portions of the chromosome located near to the erratic merle transposon.
BREEDING RECOMMENDATIONS FOR MERLE
Sometimes a dog is genetically carrying the merle factor gene, but there are no outward signs (cryptic or phantom merle). This dog could easily be inadvertently bred to another merle and result in the production of homozygous double merles, so for this reaon it is usually recommended to avoid breeding merle to brindles or any purely phaeomelanin (orange-red-gold-yellow) coat colors.
It is almost universally recommended to NEVER breed two merles together, to avoid producing defective double merles. But, unfortunately, some breeders will intentionally breed merle to merle, hoping to produce a higher percentage of merle offspring. This is sadly a misinformed idea. Others may breed merle to merle in an attempt to produce a double merle sire to use for future breeding. Such a double merle dog (even if vision or hearing impaired) will produce 100% merle offspring. There is a demand for the unusual merle color. Although this method of breeding may be considered unethical, it does happen in many breeds, by uneducated or uncaring breeders.
A merle dog should be bred to a dark colored mate, preferably black or black and tan. A dominant black dog will only produce black offspring half the time, unless he carries two copies of the dominant gene for black. Couple a black with a merle parent, who will produce merle offspring 50% of the time, and the odds just dropped to 25% for producing a blue merle from a black to merle mating...the same odds of producing a sable merle or hidden merle from such a breeding.
Black and tanpoint pattern is recessive and would breed true. If one parent is also merle, you have a 50/50 chance for producing blue merle offspring, and 50/50 tanpoint who are not merle. Recessive black would also breed true, just as black and tan does, but recessive black seems to occur only very rarely in our breed. Also, In our breed, there is a widely carried recessive gene "e" which suppresses all black pigment in the coat. This gene can double up in the offspring to hide the visual appearance of merle.
Here is part of a statement by canine geneticist Sheila Schmutz, written for the Pomeranian standard revision committee in April 2006:
"Yet another gene in the "blue" family that can cause health issues is merle. Merle can not be seen in dogs with an e/e genotype. This e/e genotype occurs commonly in Pomeranians since orange, red and white probably account for the majority of Poms. The problem is that M/M (homozygous merle) dogs are always deaf based on our studies. We recently genotyped 24 mostly white Australian Shepherds and all tested M/M (based on the Clark et al. 2006 PNAS published test) and all were deaf. A proportion of these dogs were also blind in one or both eyes since microphthalmia is another common side effect in M/M dogs. Although in many breeds it is possible to educate breeders to never breed two merle dogs together this advice is not possible to follow in Pomeranians since e/e dogs would not show the merle pattern. It would therefore be necessary instead to advise all persons who breed a merle dog to use only a black or sable mate or to have DNA testing done on their red, orange or white mate prior to breeding to be sure it did not carry merle."
Further complicating the picture is the case of breeding a merle to any dog with excessive white in the coat. Breeding merle to parti-color dogs with more than 50% white coat is advised against; especially if the merle parent also carries genes for particolor or "white spotting". It would be difficult to determine if the resulting puppies were merle or extreme piebald white-or both-without DNA testing! In either case, hearing can be affected.
DNA TEST FOR MERLE
We now have a DNA test for merle! This is a wonderful tool for breeders who choose to avail themselves of the technology. A cheek swab from the dog is sent to the lab, and they can tell you if your dog is merle or not. This test can also tell you if your dog carries one copy or two copies of the gene. Unfortunately, the US company offering this test closed its doors in March 2009. However, another company (Idexx) is poised to take over this service.
Here is the link to the Idexx website:
Unfortunately, the merle test costs close to $100, so some breeders will not o use the test, even when available. These breeders continue to try to guess the merle status of their dogs solely by appearance, noting the amount of dappling and/or white coat color.
AKC registration for Pomeranians now allows for designation of color as merle. However, this will remain an inaccurate system unless breeders DNA test all questionable offspring from their merle parents. Currently, merle is uncommon in the Pomeranian breed. However, as the pattern becomes more popular and widespread, it will more likely show up when least expected in a breeding program.
WHITE SPOTTING AND HEARING
Another pattern gene strongly associated with deafness in dogs is the "S" series-commonly known as white spotting, Irish spotting, or piebalding. Breed examples are the Bull Terrier, Samoyed, Greyhound, Great Pyrenees, Sealyham Terrier, Beagle, Bulldog, Dalmation, English Setter, Papillon and Fox Terrier.
The recessive "S" gene covers the coat color with white, and when weakly expressed allows patches of color. This process is believed to be related to the path of migration taken by the pigment cells in the embryo. During fetal development, the melanocytes (cells that produce pigment) are concentrated in the neural crest. The neural crest is the area which eventually becomes the brain and spinal cord. From the neural crest, the pigment cells migrate to the peripheral areas of the body, such as the head, legs and tail. However, sometimes the melanocytes don't travel completely over the body. Any areas where the pigment cells fail to migrate will remain white. This is why sometimes dogs will have a white spot on the end of the tail, the tips of the toes, or on the chest. If pigment-producing melanocytes fail to reach the inner ear, deafness will result.
White color on the head is often, but not always, associated with lack of inner ear pigment. Patched-colored dogs with dark eyes have been selected for in many breeds. These dogs have reduced incidence of hearing impairment when compared to dogs which are almost totally white.
Not all breeds with the "S" -extreme piebald pattern have been reported to be affected with deafness, and there is also inherited deafness in several breeds which is totally unrelated to coat color.
The only way to know for sure if your dog is hearing impaired is through a Brainstem Auditory Evoked Response Test, or BAER test. This testing is only available at select facilities and can be expensive. It need only be performed once in a dog's lifetime (preferably at a young age) to rule out hereditary hearing impairment. A complete list of BAER testing facilities can befound on Dr. George Strain's website: www.lsu.edu/deafness/deaf.htm. Selective breeding for hearing dogs, as evidenced by normal BAER testing, can reduce the incidence of deafness in dogs, including those with high risk factors.
Other health concerns associated with lack of pigmentation are skin allergies, skin cancer and sunburn, demodicosis (immune-mediated hair follicle mange), follicular dysplasia (coat which is dull and breaks), reduced fertility and sterility, and photo-induced epilepsy.
CONCERNS RELATED TO THE PRODUCTION OF HANDICAPPED DOGS
Some animal welfare specialists endorse humane euthanasia for all dogs deaf in both ears (bilaterally deaf). Hearing can not be accurately assessed before 5 weeks of age. At this age, euthanasia can be an agonizing decision for any caring, ethical breeder.
Deaf or blind dogs require special placement arrangements. A breeder is morally and ethically responsible for care and placement of handicapped dogs. We may see in the future an increase in handicapped Pomeranians turned over to rescue or to shelters. As there are always so many healthy dogs in shelters and rescue who cannot find homes, the odds of finding homes for handicapped dogs are even greater. Support for rescue from our parent and regional clubs will likely need to increase.
WHERE DO WE GO FROM HERE?
Many people have voiced concern that the merle pattern has only recently been introduced to our breed, through crossbreeding with merle dogs of other breeds. However, all breeds have been created through such methods at some point in their history. The concern is not so much HOW the pattern appeared in our breed, but rather WHERE it will take us in the future.
APC is the parent club for the Pomeranian, and as such, should serve the breed with the commitment and resonsibility that the word "parent" implies. It is a responsibility of a parent club to discourage breeding that is known to increase the probability of dogs with health problems. This can be attempted through education of breeders and the public. Such education can be provided on the club website or through the person designated as health and genetic liaison. Education should include factors to consider in selecting a canine companion, as well as methods of developing a breeding program with an emphasis on producing healthy offspring. Public awareness of health issues may eventually create an increased demand for dogs bred as much for good health as for physical beauty.
Many breed clubs promote responsible breeding through the breed standard Itself. (For instance, our Pomeranian standard stipulates that eyes must be dark).
Guidelines can also be provided through a breed club's code of ethics. The guidelines might contain recommendations such as:
1. Yearly CERF testing for all breeding stock, particularly merles. (Clinics should be made available at national and regional specialties).
2. BAER testing should be performed once in a dog's lifetime prior to breeding. This is especially important in merle and extreme piebald patterns.
3. All dogs from merle litters be sold only to knowledgeable breeders or transferred to new owners after being spayed or neutered. This will help prevent uneducated breeding and the production of handicapped dogs.
4.Vision and hearing health should be guaranteed in all puppy contracts, Dogs so affected should be readily taken back by their breeder at any point in their lifetime.
5. Any puppies from a litter with a merle parent who do not appear to be merle should be DNA tested to determine merle status, (unless of course they are going to be sterilized, in which case DNA merle testing would not be necessary).
As breeders, we need to avoid the ostrich syndrome. Let's not stick our heads in the sand and hope problems will go away....they might get worse if we do!
STUDY!! We should learn all we can about genetics and heredity-of all breeds, not just our own. The other guy's problem today may well be ours tomorrow.
Our breed already has some health concerns. It would seem unwise to encourage an increase in defects purely for the sake of novel color patterns. Through education and personal responsibilty, risky breeding practices can be minimized. We should strive to produce puppies blessed with the gifts of normal vision and hearing. If this article serves to prevent the birth of just one handicapped dog through heightened breeder awareness, it will have been worthwhile.
PIGMENT-RELATED DISORDERS IN HUMANS
One in 17,000 persons in the USA has some type of albinism, as indicated by little or no pigment in their eyes, skin, or hair. People with albinism always have problems with vision. This results from abnormal development of the retina and abnormal patterns of nerve connections between the eye and brain. It is the presence of these eye problems which defines the diagnosis of albinism. Albinism may be limited to the eyes or involve hair and skin to varying degrees.
Waardenburg syndrome is an inherited disorder in humans characterized by varying degrees of hearing loss and changes in skin and hair pigmentation. This syndrome was named for the Dutch ophthalmologist who noticed that people with pigmentation abnormalities of the hair, eyes and skin, often had hearing impairment.Those with Waaardenburg syndrome may have two differently colored eyes, one brown and one blue. Sometimes, one eye has two different colors. Others may have unusually brilliant blue eyes. People with Waardenburg syndrome also have distinctive hair coloring, such as a patch of white hair near the forehead, or they might have white patches of skin. Waardenburg's is associated with other birth defects such as spinal or intestinal disorders or cleft palate.
After doing this research, I finally realized why I reach for sunglasses each and every time I go outdoors on a sunny day! I have blue eyes, and cannot tolerate the bright light. My eyes lack sufficient light-blocking pigment!
Sincere thanks to TAMU's Leigh Anne Clark, PhD for answering my many questions. You are wonderful!
Thanks to Sheila Schmutz, PhD for her expert advise on canine color genetics:
Thanks to JP Yousha for her color genetics expertise! Your input was invaluable.
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