M E R L E . . . P A T T E R N

as it's known in the Rat Terrier Breed


 

Where to start ... where to start! This is probably the most controversial pattern of any as it is associated with the Rat Terrier.
Many are going to say that a Rat Terrier with a MERLE pattern is not a true Rat Terrier but I'll have to disagree.
The Rat Terrier evolved in the 1970's through the 1990's from the crossing of the Chihuahua, Miniature Pinscher, Toy Fox, Italian Greyhound
and the Beagle.  Some now say it also evolved from the Whippet, Corgi and Dachshund.  My research shows the Whippet actually evolved from the Rat Terrier and not the other way around.  The Corgi and Dachshund, if part of the breed, would of also contributed the Merle gene.  I know that the Chihuahua was an accepted cross for the breed back in the 80's and 90's so I'll concentrate on the Chihuahua as being the contributing factor of the merle gene.

The MERLE pattern does exist in the Chihuahua and is at the time of this being written (*2006) recognized by the AKC therefore making it a
logical conclusion that it would exist in Rat Terriers.
There are FEW of them around apparently because few breeders crossed the Merle Chihuahua with anything to produce them.
We might want to note that one parent MUST be Merle to produce Merle pups.
It seems that there are some Rat Terrier breeders, one in particular, out there on the web today that have made the decision to try to
discredit those who breed the merle Rat Terriers.  To those I say that you should check your facts and get them straight before
you try educating others on things that you obviously know nothing about.
 The merle Rat Terrier is every much a PUREBRED Rat Terrier as any others in existence today.  As with any breed the Merle was
introduced into the breed by the Chihuahua back in the 1970's or possibly before but my research has gone back as far as
1970 to documented Rat Terriers that were merle in pattern.  Since there is no way to prove what ANY Rat Terrier of today is made
up of as there is no test as of yet for a purebred Rat Terrier geneotype since they are composed of so many different breeds. 
 UKCI, the founder of the Rat Terrier as a purebred breed, allowed the crossing of what were considered the few documented
Rat Terriers with Chihuahua, Miniature Pinscher, Italian Greyhound and the beagle for years in order to increase the gene pool
and downsize the breed as many wanted smaller type Rat Terriers.  During this period of time a merle Chihuahua was apparently
used in a few (*very few) breeding programs which produced the start of the merles that we see today.   If the merle Rat Terrier
is not considered to be a part of the breed and a purebred Rat Terrier then neither can we acknowledge the Toys and/or Miniatures
as they were developed along the same lines of the Merle Rat Terrier.  If the merle Rat Terrier can not be acknowledged as a
purebred Rat Terrier then neither can the blues, browns (*chocolates) or fawns as those colors were produced from crossing
the few documented Rat Terrier with other breeds as well.  If the merle Rat Terrier can not be considered
part of the developed breed then neither can any of the dogs in existence today that can not trace it's documented lineage back to the
England mines which means there are NO purebred Rat Terriers as nobody can trace their Rat Terrier back to the beginning as we
do not know, for certain, the history of the Rat Terrier or it's development as nobody has ever produced documented information to
give us insight as to exactly how the Rat Terrier came to be.  We do have our theories and if you asked 100 people they would have
100 slightly different if not completely different opinions of how the Rat Terrier came to be. 

 

Don't let yourself be fooled by those who would deceive you into believing that the Merle Rat Terrier
oes not have it's place in the Rat Terrier world and it's heritage and history as it does, has and will continue to be.

 

Now that we've got the Myth, lies, theories and deceptions of the purebred status out of the way, let's move on to the lies,
deceptions and completely untruths of the health of the merle gene itself.  The single merle gene has no more health issues than
that of the solids or piebald's.  Those that claim otherwise are totally misinformed and from what I have seen from their websites
apparently need a lesson in 101 genetics as their claims hold no logical basis and are easily disproved.  
I've been breeding merles since 1986 in what would be considered large numbers for
the merle world and have yet to have one that had any hearing, eye or skin problem.
  It's beyond ridiculous to even think that the merle pattern has within it's power to cause demodectic issues but there are
 a couple of websites out there that are trying to lead you to believe just such an occurrence happens.  If a skin issue is involved
then they need to look within their bloodline and not blame the merle gene for something that it has no way of being involved in.
 The 'single' merle gene does not cause any health issues and can be safely bred to a non-merle mate.
Anybody with any knowledge at all should know that you never breed 2 Merles together as this "MIGHT" produce a 'lethal' double merle.
It's not hard to deduce for the majority of folks that have a little common sense that you simply don't breed 2 merles together thus you
avoid all of the pitfalls of the 'double merle'.  There is a website out there who is trying diligently to say that the majority of double merles
have issues which is blatantly false and for all conceived purposes a 'LIE' as it doesn't take but 2 minutes of research to show that only
25% of doubles have any issues which is 1 in 4.  Let me state that again 25% of double merles, on the average, have some kind of issue,
whether that be reduced life-span to dying in the uterus. 

 

There is even several websites out there with the story of a 'single' merle gene producing defective, deformed offspring.  This is
 strictly BIAS  'opinion' in the highest form as they have NO RESEARCH or lab work to back their claim as there as been NONE.
More than likely they had recessive defective genes in the bloodline that matched up with other recessive genes causing
major issues with the offspring that had NOTHING to do with the merle gene.  A little common sense
goes a long way in dog breeding but I guess trying to prove you know what your talking about with lies, deception,
 untruths and partial knowledge works better than doing the research, running the test and finding out 'exactly'
what happened and telling the 'facts' and not 'opinion' as it serves them to 'try' to establish their basis for their opinion.
I often wonder how they explain the other thousands of puppies that are born that are NOT merle that are blind,
deaf, deformed - etc that they can't say is the fault of the merle gene.  Guess poor genetics, poor breeding and
unknowledgeable breeder doesn't hold a place in their shallow brains and tunnel vision eyes.
I tested over 25 merle dogs in the beginning as I wanted to be sure that I was breeding healthy, genetically sound dogs
 with no issues.  Not one single one of my merle dogs had/has any issues with hearing, eye sight, life-span, reproduction
or otherwise so I challenge those that say the 'single' merle is a defective gene to PROVE it or keep their "OPINIONS" to themselves.
I have been breeding merles for over 25 years and have produced and sold my fair share and to date, I have yet to have a
puppy buyer call me and tell me that any that I have ever sold had any issues as well and it doesn't take being in the dog business
long to know that if you sell a dog with an issue that your gonna hear about it.
Last but not least the 'MONEY" issue - I guess I've missed where the merles sell for any more than any
other pattern of Rat Terrier.  Mine are all the same price and have been for years - You bet that at one time they were more as
supply and demand allowed it just as the blues, browns and fawns all had their glory days of higher prices at one time.
If those that complain about prices being higher for one dog than it is for another could get higher prices for theirs, they would do it in a
heartbeat and if they say otherwise then they are flat out lying.  If a dog from a show line is supposed to be worth more
and we're seeing more and more issues with dogs from show lines then why wouldn't a healthy one with a gorgeous coat be
worth more as well.  I'll take health over Red Line Pedigrees, color and pattern any day.
I call a spade a spade and don't feel the need to sugar-coat everything.

 

The Piebald gene has proven to cause deafness if the white concentrates around the ear area but I fail to see these same nay-sayers trying to
do away with the Piebald gene in the breed.  Why is that - maybe because they don't have an issue with Piebald but have a personal
aversion to Merle and it is just that a PERSONAL issue and nothing whatsoever to do with the heritage or any other factor ... period.
 
Here is the TRUTH about the Merle Genetics
From the Man, himself who is so often misquoted in others attempts to deceive the dog world about Merle Genetics::
George M. Strain, PhD
"Recent issues of Top Notch Toys have printed dialog about the merle gene,... . One particular article (1) cited research of mine (2) with an incorrect interpretation that I wish to correct. In addition, I would like to provide unbiased up-to-date information on the merle gene that may inform and clarify the debate on this issue. I have been performing research on hearing and deafness since the late 1980's, and am identified as a leading authority on deafness in dogs, so I am well positioned to provide this information. I should point out that publications and writings of mine from past years discussing the merle gene no longer represent my opinion, as recent research has led me to change my position.
The above cited article contained the statement that According to Dr. George Strain merle and piebald dogs with blue eyes are 50% more likely to be deaf.@ The research from which this was drawn only applied to the piebald gene and only applied to the Dalmatian breed, where blue eyes and deafness are a wide-spread problem (30% of US Dalmatians are deaf in one or both ears). My research did not apply to dogs with merle, and I am unaware of any study examining this issue using adequate numbers of dogs and dogs from breeds other than Dachshund, where the published studies have limitations (see below).
Two pigment genes are associated with deafness in dogs: piebald (s) and merle (M). Piebald, which is present in Dalmatians, bull terriers, cocker spaniels, Jack Russell terriers, Chihuahuas and others (RAT TERRIERS), is a recessive gene. There are three recessive alleles for piebald: Irish spotting (si), piebald (sp), and extreme white piebald (sw); dogs that have uniform color without white carry the dominant allele (S). The piebald gene produces areas of white by suppressing pigmentation cells (melanocytes). Merle, which is present in Shetland sheepdogs, Australian shepherds, Dachshunds, Great Danes and others, is a dominant gene. Merle produces a color pattern where patches of color are diluted or absent (white); animals homozygous with the recessive allele (mm) have solid color. Dogs with piebald must be homozygous to have areas of white, while merles can be either heterozygous (mM) or homozygous (MM). There is no evidence to suggest that dogs carrying both the piebald and merle genes have an increased likelihood of deafness.
Much of the literature on merle in the past focused on problems seen in homozygous merles and in breeds where the merle gene can produce dramatic effects B in some cases including deafness, blindness and microphthalmia, and sterility. Even heterozygous dogs in these breeds can have less serious visual and auditory deficits. This indeed happens with some breeds, but unfortunately many people have taken this truth and extrapolated it to apply to all breeds carrying the merle gene, which is not true. For example, dogs in the Catahoula breed can be homozygous merle without any of these health defects, and heterozygotes do not seem to be affected. Until recently it was not possible to even distinguish between mM and MM merles in some breeds.

Since not all breeds carrying the merle gene experience the deleterious effects, it is incautious to proclaim that the presence of this pattern in a breed will be injurious to the breed without first investigating whether deaf or blind dogs result from breeding heterozygous merles. Are there any known deaf or blind merle Chihuahuas? If so, are they heterozygous or homozygous? In many breeds carrying merle, breeders know not to breed homozygous merles, and visual and auditory deficits do not seem to be a problem in the heterozygotes. Studies have examined auditory function (3) and visual function (4) in heterozygous and homozygous dappled (merle) Dachshunds, as described in several writings by Dr. Malcolm Willis. These studies, from geographically and numerically restricted populations, found hearing loss and deafness and visual abnormalities, but only examined small numbers of dogs B 38 in the first study and 18 in the second. Dappled Dachshunds, when carefully bred to avoid MM, do not appear to have deafness or blindness in the general population, so one must be careful to not raise alarms at the presence of merle in a breed until experience shows that a true problem exists.
A large leap in understanding merle occurred when Clark and Murphy of Texas A&M University identified and sequenced the canine gene for merle in 2006 (5). The gene, named SILV, (also known as Silver in mice) plays a role in pigmentation in skin, eye, and ear. Dogs with the merle phenotype have a short piece of DNA inserted into this gene B a DNA modification known as a short interspersed element (SINE). This work was performed with Shetland sheepdogs, then confirmed in merles from eleven other breeds, including chihuahua. The sequence of the SINE was the same in all breeds, suggesting that all breeds in the study shared a common ancestor. The merle SINE insertion has three components: a head, a body and a tail; the latter contains a long string of repeated adenine nucleotides (polyA). For a dog to show the merle phenotype, it must have both the SINE insertion and a polyA tail that is of sufficient length (90-100 adenine repeats). Some merle-merle breedings produce homozygous merles called cryptic because they don=t show the merle phenotype, and when bred they do not produce any merle offspring. It turns out that the polyA tail in cryptic merles has been truncated to 65 or fewer adenine repeats. So, the merle gene phenotype can revert to the non-merle in one generation. In the same way, it is theoretically possible for the polyA tail length to increase from genetic processing error, spontaneously producing a merle (5,6). The likelihood of this possibility is unknown but probably low.
It has been suggested that merle appeared in the Chihuahua breed from a cross to another breed, such as the Dachshund. Others have suggested that the gene has been present for many generations, but that the pigmentation pattern was incorrectly described, such as blue and tan or black and silver. A single event of the first possibility might still make it hard to explain all of the merle Chihuahuas now in existence. Regardless of the source of merle in the breed, to my knowledge there is no data at this time to suggest that merle Chihuahuas are prone to visual or auditory problems. I would encourage the breed organization investigate the prevalence of visual and auditory disorders in merle Chihuahuas prior to making decisions affecting the breed standard."


 
The Truth About Merle Research from the leading Authority of Deafness in Dogs
 
Dr. Strain's research appeared to link the congenital hereditary deafness to both Merle and Piebald
in breeds other than the Rat Terrier or TRT.   He clearly states that we can not take studies done from one
breed and apply it to others. 
He also states " Our data analyses suggest that the inheritance involves more than one gene: M or s and another gene
that modifies how strongly the first gene acts." 
Of 48 single merles tested only 1 was found to be deaf and that was a Great Dane that was also
piebald in pattern as well.  Dr. Strain also continues to say testing  "suggests a breed difference  for the impact of the merle gene
on hearing status", which clearly means we can not take test performed on other breeds and apply them to ones we choose to in an
attempt to lead others astray. 
Dr. Strain also references the "2006" testing and makes it known that only 1 out of 48 merles

in the study was deaf and it was a Great Dane which also carried the Piebald gene.  None, Nata,

Zip, Zero Rat Terriers or Teddy Roosevelt Terriers were part of the 2006 Study.

Those trying to use the 2006 Study to prove their right about Merle in Rat Terriers should be

ashamed of themselves for trying to lead others astray.  I could take the PIEBALD results and blow the

Rat Terrier breed genetics too heck in a hand basket but that would be ludicrous just as what

others are doing with the ridiculous attempts to harm the Merle breeders.

Will we do away with the Rat Terrier and TRT breed totally? 

If we're going to skew the results of Merle then we also

must take the results of the Piebald research.  I wonder "who breeds Piebald Rat Terriers" but conveniently

overlooks the piebald research results in relation to it's genetic quirks?

 
 
The Truth About Merle Research from the leading Authority of Deafness in Dogs
 
Dr. Strain's research appeared to link the congenital hereditary deafness to both Merle and Piebald
in breeds other than the Rat Terrier or TRT.   He clearly states that we can not take studies done from one
breed and apply it to others.  He also states " Our data analyses suggest that the inheritance involves more than one gene:
M or s and another gene that modifies how strongly the first gene acts." 
Of 48 single merles tested only 1 was found to be deaf and that was a Great Dane that was also
piebald in pattern as well.  Dr. Strain also continues to say testing  "suggests a breed difference  for the impact of the merle
 gene on hearing status", which clearly means we can not take test performed on other breeds and apply them to ones we
choose to in an attempt to lead others astray. 
Dr. Strain also references the "2006" testing and makes it known that only 1 out of 48 merles
in the study was deaf and it was a Great Dane which also carried the Piebald gene.  None, Nata,
Zip, Zero Rat Terriers or Teddy Roosevelt Terriers were part of the 2006 Study.
Those trying to use the 2006 Study to prove their right about Merle in Rat Terriers should be
ashamed of themselves for trying to lead others astray.  I could take the PIEBALD results and blow the
Rat Terrier breed genetics too heck in a hand basket but that would be ludicrous just as what
others are doing with the ridiculous attempts to harm the Merle breeders.
Will we do away with the Rat Terrier and TRT breed totally? 
If we're going to skew the results of Merle then we also
must take the results of the Piebald research.  I wonder "who breeds Piebald Rat Terriers" but conveniently
overlooks the piebald research results in relation to it's genetic quirks?

___________________________________________

Deafness and the Merle Gene

George M. Strain, PhD
Professor of Neuroscience
Louisiana State University, Baton Rouge, LA
           
            Of all the domestic species, the canine has the greatest variation in size, shape, and skin pigmentation pattern.1
Results from classical genetic studies in the last century identified at least ten20genetic loci that determine coat color and pattern,
represented by the letters A, B, C, D, E, G, M, P, S, and T.2 Two of these genes, S (piebald) and M (merle), have been linked to the
appearance of congenital hereditary deafness. The S series has one dominant and three recessive alleles: the dominant S allele produces a
solid coat color, while the recessive alleles si (Irish spotting), sp (piebald), and sw (extreme piebald) produce increasing amounts of white in the
coat and skin. The Dalmatian breed is homozygous for sw and is the breed with the highest prevalence of deafness: 30% are deaf in one or both ears.
Other breeds carrying recessive piebald alleles with deafness problems include the bull terrier, English setter, English cocker spaniel, Australian
cattle dog, and Jack Russell terrier.
            The second pigmentation gene associated with deafness is merle. The dominant allele M acts on uniform pigmentation to produces
an alternating pattern of dark versus light that is also known as dapple. The recessive allele produces uniform pigmentation when the dog
is homozygous (mm). Heterozygous merle (mM) in an otherwise black dog produces a blue merle, and in an otherwise brown=2 0dog produces
a red merle. Dogs homozygous for the dominant allele (MM) can be mildly affected to the naked eye or severely affected, depending on
breed and even varying within a breed. Severely affected MM individuals are often nearly all white, deaf, sterile, and blind or affected by
various visual abnormalities. Merles are commonly seen in the collie, border collie, Australian shepherd, Shetland sheepdog,
Cardigan Welsh corgi, dachshund, and Great Dane breeds; other breeds less commonly known to carry merle are the Chihuahua,
American pit bull terrier, American Staffordshire terrier, Beauceron, Catahoula leopard dog, Koolie, poodle, Pyrenean shepherd,
Old English sheepdog, American cocker spaniel, Pomeranian, Hungarian Mudi, Norwegian dunkerhound, and others.
 Many of the breeds that carry merle also carry piebald. Whether it linked to S, M, or other causes, congenital deafness has been identified
in nearly 90 breeds,3 nearly all of which carry piebald, merle, or both. While we know that the piebald gene is inherited as a simple recessive
and the merle gene as a simple dominant, the inheritance of deafness resulting from either gene does not appear to be inherited in a simple
Mendelian manner – I’ve bred deaf Dalmatian to deaf Dalmatian and gotten bilaterally hearing puppies. Our data analyses suggest that the
inheritance involves more than one gene: M or s and another gene that modifies how strongly the first gene acts.
            Relatively few studies of the merle gene have been published, most coming from studies of a breeding colony of merle dachshunds
kept at a university in Hanover, Germany. One study4 examined auditory function in the animals, and several (e.g. reference 5) examined
visual function. From these limited studies of an inbred population in one breed, subsequent authors have,
unfortunately, extrapolated the reported findings to apply to all merle-carrying breeds. Current work in our and other
laboratories and the experiences of many breeders have shown that the actions of merle have usually been over-stated.
Reetz et al.4 reported hearing results for 38 dachshunds (Tekels in German): 11 double merles, 19 single merles, and 8 non-merles.
They found hearing loss – slight to total, unilateral or bilateral – in 54.6% of double merles, in 36.8% of single merles, and in none of
the non-merles. Hearing was tested using the brainstem auditory evoked response (BAER), determining the threshold to click stimuli under
sedation. Any threshold above 20 dB was considered to be abnormal, not because that is an accepted standard, but because one of their
non-merle dogs had a 20 dB hearing threshold. Only one dog – a double merle male – was totally deaf in both ears (threshold > 90 dB) and
none of the dogs were totally deaf in only one ear (unilaterally deaf). Looked at this way, true bilateral deafness occurred in 9.1% (1/11) of the
double merles and 0% of the single merles.
            How can the reported hearing loss in the remaining single and double merles be explained? The pigment-associated deafness
seen with the piebald and merle patterns typically presents as total deafness in one or both ears, based on all of the histological
studies that have been reported, so the partial hearing loss reported by Reetz is not likely to be genetic or associated with the merle gene.
Instead, it most likely reflects a combination of poor aural h ygiene (dirty ear canals), middle ear infections, and noise-induced hearing trauma.
The noise level in institutional kennels is notoriously high, and exposure to high noise levels produces cumulative hearing loss. Dogs in large
kennels also usually do not receive regular ear cleaning, leading to build up of excess cerumen and infections, both of which muffle the sound
reaching the inner ear. Interestingly, of the 15 “hearing impaired” ears with thresholds between 25 and 50 dB, only 3 were in males. Perhaps
differences in kennel housing for females exposed them to greater noise levels in the whelping kennels. Regardless, the hearing loss reported
in these dachshunds that can be attributed to a genetic cause is much lower than stated in the published English abstract of this German publication.
            In 2006 the gene responsible for the merle pattern in dogs was identified and sequenced6 and a commercial DNA test is now available
to determine whether a dog is a single or double merle. In an unpublished study performed by myself and these investigators at Texas A&M
University,7 70 merle dogs from five b reeds (Shetland sheepdog, Australian shepherd, collie, Great Dane, and Catahoula leopard dog) had
BAER hearing tests performed and merle genotype determined by DNA tests. Of 22 double merles, 8 were bilaterally deaf (36%) and 2
were unilaterally deaf (9%). Of 48 single merles only one was unilaterally deaf (2%), a Great Dane that also carried the piebald gene,
raising a questions as to the cause for the deafness, and none was bilaterally deaf. Based on Reetz’s study about one third of the single merles
would have been expected to have significant hearing loss.
            An interesting finding came from our group of 70 dogs: 15 of the double merles were Catahoulas, but only 4 of them were deaf in one
or both ears (27%), while 86% of the double merles in the other breeds (Shetland sheepdog, Australian shepherd, collie) were deaf. This
suggests a breed difference for the impact of the merle gene on hearing status, which may not be surprising since most double merle
Catahoulas are heavily pigmented compared to double merles in the other breeds. We are continuing to test additional dogs to further
document and understand these differences.
            What do experienced breeders in merle-carrying breeds have to say? I cannot present any numbers because I have not done formal
surveys, but I’ve repeatedly heard from long-time breeders who say that they seldom if ever get deaf or blind dogs from breeding merle merle,
especially in the dachshund and Catahoula breeds. There is no denying that such outcomes do occur, especially, it would seem, in the collie-type
breeds, but we do not at this time know the determinants or conditions that produce deaf or blind dogs.
 
            What is to be done, then, about the merle gene? It seems clear that merles in some breeds, especially double merles, present a problem.
In other breeds the problem is significantly less problematic. It might be said that recent efforts in several national breed clubs to ban merle
completely from the breed standard is throwing out the baby with the bath water. Others would argue that the production of even small numbers
of puppies with auditory or visual defects is an adequate reason to eliminate the pattern. If we completely understood how merle works and what
determines deaf or blind merle puppies, it might be easier to assert with confidence the right way to proceed. Leaving aside the ascetics of
the merle pattern appearance, a matter of personal preference, we can say with confidence based on current data
that single merles have a very low likelihood of deafness. Double merles in some breeds – Catahoula, dachshund –
 have a finite but still low probability of being deaf, while double merles in some other breeds – the collie-type breeds – have a high likelihood of
producing deaf. Knowing how strongly double merle dogs are impacted within a breed should provide guidance to breeders on whether they
should avoid breeding double merles. Researchers will provide some guidance, but unfortunately the studies will take some time. In the mean time
it seems prudent to delay making difficult-to-reverse changes in breed standards based on limited information. In most cases the
breed standards have been in place for many decades; a few more years of waiting won’t bring about the end of the world while we
develop a better understanding of merle.

References

 
1.         Ostrander EA, Wayne RK. 2005. The canine genome. Genetic Research 15: 706-1716.
2.         Little CC. 1957. The Inheritance of Coat Color in Dogs. New York: Howell Book House, 194 pp.
3.         Str ain GM. 2007. Deafness in Dogs and Cats. http://www.lsu.edu/deafness.deaf.htm.
4.         Reetz, I, Stecker, M, & Wegner, W. 1977. Audiometrische Befunde in einer Merlezucht [Audiometric findings in dachshonds (merle gene carriers)]. Deutsche Tierärztliche Wochenschrift 84, 273-277.
5.         Klinckmann G, Koniszewski G, & Wegner W. 1986. Light-microscopic investigations on the retinae of dogs carrying the merle factor. Journal of Veterinary Medicine A 33:674-688.
6.         Clark LA, Wahl JM, Rees CA, & Murphy KE. 2006. Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proceedings of the National Academy of Sciences103:1376-81.
7.         Clark, LA, Wahl JM, Rees CA, Strain GM, Cargill EJ, Vanderlip SL, & Murphy KE. 2007. Canine SINEs and their effects on phenotypes of the domestic dog. In: P. Gustafson, ed. Genetics of Disease (in press).
The Author
George M. Strain is a professor of neuroscience at the LSU School of Veterinary Medicine, where he has studied deafness in dogs and cats for
over twenty years. His training is in electrical engineering, biomedical engineering, physiology, and neurology.
 
Merle Strain
George M. Strain, PhD

Recent issues of Top Notch Toys have printed dialog about the merle gene, especially relating to its presence in the Chihuahua breed. One particular article (1) cited research of mine (2) with an incorrect interpretation that I wish to correct. In addition, I would like to provide unbiased up-to-date information on the merle gene that may inform and clarify the debate on this issue. I have been performing research on hearing and deafness since the late 1980's, and am identified as a leading authority on deafness in dogs, so I am well positioned to
provide this information. I should point out that publications and writings of mine from past years discussing the merle gene no longer represent my opinion, as recent research has led me to change my position.
The above cited article contained the statement that According to Dr. George Strain merle and piebald dogs with blue eyes are 50% more
likely to be deaf.@ The research from which this was drawn only applied to the piebald gene and only applied to the Dalmatian breed, where blue eyes and deafness are a wide-spread problem (30% of US Dalmatians are deaf in one or both ears). My research did not apply to dogs with merle, and I am unaware of any study examining this issue using adequate numbers of dogs
and dogs from breeds other than Dachshund, where the published studies have limitations (see below).
Two pigment genes are associated with deafness in dogs: piebald (s) and merle (M). Piebald, which is present in Dalmatians, bull terriers, cocker spaniels, Jack Russell terriers, Chihuahuas and others, is a recessive gene. There are three recessive alleles for piebald: Irish spotting (si),
piebald (sp), and extreme white piebald (sw); dogs that have uniform color without white carry the dominant allele (S). The piebald gene produces areas of white by suppressing pigmentation cells (melanocytes). Merle, which is present in Shetland sheepdogs, Australian shepherds, Dachshunds, Great Danes and others, is a dominant gene. Merle produces a color pattern where patches of color are diluted or absent (white); animals homozygous with the recessive allele (mm) have solid color. Dogs with piebald must be homozygous to have areas of white, while merles can be either heterozygous (mM) or homozygous (MM). There is no evidence to suggest that dogs carrying both the piebald and merle genes have an increased likelihood of deafness.
Much of the literature on merle in the past focused on problems seen in homozygous merles and in breeds where the merle gene can produce dramatic effects B in some cases including deafness, blindness and microphthalmia, and sterility. Even heterozygous dogs in these breeds can have less serious visual and auditory deficits. This indeed happens with some breeds, but unfortunately many people have taken this truth and
extrapolated it to apply to all breeds carrying the merle gene, which is not true. For example, dogs in the Catahoula breed can be homozygous merle without any of these health defects, and heterozygotes do not seem to be affected. Until recently it was not possible to even distinguish
between mM and MM merles in some breeds.

Since not all breeds carrying the merle gene experience the deleterious effects, it is incautious to proclaim that the presence of this pattern
in a breed will be injurious to the breed without first investigating whether deaf or blind dogs result from breeding heterozygous merles. Are
 there any known deaf or blind merle chihuahuas? If so, are they heterozygous or homozygous? In many breeds carrying merle, breeders
know not to breed homozygous merles, and visual and auditory deficits do not seem to be a problem in the heterozygotes. Studies have examined auditory function (3) and visual function (4) in heterozygous and homozygous dappled (merle) Dachshunds, as described in several writings by Dr. Malcolm Willis. These studies, from geographically and numerically restricted populations, found hearing loss and deafness and visual abnormalities, but only examined small numbers of dogs B 38 in the first study and 18 in the second. Dappled Dachshunds, when carefully bred to avoid MM,
do not appear to have deafness or blindness in the general population, so one must be careful to not raise alarms at the presence of merle in a breed until experience shows that a true problem exists.
A large leap in understanding merle occurred when Clark and Murphy of Texas A&M University identified and sequenced the canine
gene for merle in 2006 (5). The gene, named SILV, (also known as Silver in mice) plays a role in pigmentation in skin, eye, and ear.
Dogs with the merle phenotype have a short piece of DNA inserted into this gene B a DNA modification known as a short interspersed element (SINE). This work was performed with Shetland sheepdogs, then confirmed in merles from eleven other breeds, including Chihuahua.
The sequence of the SINE was the same in all breeds, suggesting that all breeds in the study shared a common ancestor. The merle SINE insertion has three components: a head, a body and a tail; the latter contains a long string of repeated adenine nucleotides (polyA). For a dog to show the merle phenotype, it must have both the SINE insertion and a polyA tail that is of sufficient length (90-100 adenine repeats). Some merle-merle breedings produce homozygous merles called cryptic because they don't show the merle phenotype, and when bred they do not produce
 any merle offspring. It turns out that the polyA tail in cryptic merles has been truncated to 65 or fewer adenine repeats. So, the merle gene
phenotype can revert to the non-mer le in one generation. In the same way, it is theoretically possible for the polyA tail length to increase
from genetic processing error, spontaneously producing a merle (5,6). The likelihood of this possibility is unknown but probably low.
It has been suggested that merle appeared in the Chihuahua breed from a cross to another breed, such as the Dachshund. Others have suggested
that the gene has been present for many generations, but that the pigmentation pattern was incorrectly described, such as blue and tan or black
and silver. A single event of the first possibility might still make it hard to explain all of the merle Chihuahuas now in existence. Regardless of the source of merle in the breed, to my knowledge there is no data at this time to suggest that merle Chihuahuas are prone to visual or auditory problems. I would encourage the breed organization investigate the prevalence of visual and auditory disorders in merle Chihuahuas
prior to making decisions affecting the breed standard.

1. Lambert G. 2006. Chihuahuas - Any color (breed) marked or splashed?? Top Notch Toys 22(4):116.
2. Strain GM. 2004. Deafness prevalence and pigmentation and gender associations in dog breeds at risk. The Veterinary Journal 167(1):23-32.
3. Reet z, I., Stecker, M., & Wegner, W. 1977. Audiometrische Befunde in einer Merlezucht [Audiometric findings in dachshunds (merle gene carriers)]. Deutsche Tierärztliche Wochenschrift 84(7):273 277.
4. Klinckmann G, Koniszewski G, Wegner W. 1986. Light microscopic investigations on the retinae of dogs carrying the Merle factor. Journal of Veterinary Medicine A 33(9):674 688.
5. Clark LE et al. 2006. Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proceedings of the National Academy of Sciences 103(5):1376 81.
6. Cordaux R, Batzer MA. 2006. Teaching an old dog new tricks: SINEs of canine genomic diversity. Proceedings of the National Academy of Sciences 103(5):1157 8.


PIEBALD
Does this apply to the Rat Terrier Breed?  I personally don't think so just as I do not believe any Merle
research to date reflects the Merle genetics in the Rat Terrier or TRT.
Have their been PIEBALD Rat Terriers and TRT's that have flunked their Bare and Eye Test?
Of course they have but that doesn't mean we are going to eliminate them all - Does it?
Are we going to associate Luxtating Patella's and Bad Hips to a pattern next?
Let's stick with what we know and not make assumptions or be led astray by those who like to attempt to cause trouble for others.

PIEBALD RESEARCH on Eye Conditions associated with the Piebald gene:

Volume 38 - 1952
GENETICS: DUNN AND MOHR
AN ASSOCIATION OF HEREDITARY EYE DEFECTS WITH WHITE SPOTTING
By L. C. DUNN AND JAN MOHR*  COLUMBIA UNIVERSITY
Communicated August 20, 1952
In a paper in these PROCEEDINGS1 an asymmetrical effect on eye color  in the house mouse was ascribed to a new mutant gene, ruby eye (ru).
 It was noted, however, that almost all mice in which the two eyes differed in color, referred to as heterochromia iridis, showed piebald spotting.
This connection between eye color variation and spotting has now been studied further with the result that the eye color modification has been
shown to be due to specific defects of the iris, which in turn appear to be effects of a mutant gene, s, for piebald, which has long been
known in laboratory mice.
A strain of ruby-eyed mice without spotting has been observed for over twenty generations and no case of heterochromia has been seen.
Evidence for the association of iris defects and spotting will be given below, together with a preliminary description of the iris abnormalities derived
from gross examinations with an ophthalmoscope. All animals in these experiments were thus observed at the time of weaning (3-4 weeks of
age) and the diagnosis was in many cases confirmed by later examinations.
A careful microscopic study of the defects remains to be made.
Anisocoria in Piebald Mice.-Many of the mice in our inbred strain
An (= aniridia: black-and-tan silver ruby piebald aYa BB ruru sisi ss)
show noticeable differences from the normal ruby eye color. One or both eyes may be light ruby or pale pink. When these variant eyes are examined
with an ophthalmoscope the commonest finding is an enlarged pupil (anisocoria) varying all the way from slight enlargement to what
appears to be complete absence of the iris (aniridia). Enlargement of the pupil is generally accompanied by reduction of the amount of pigment in
the iris with consequent approach to the pink color of unpigmented eyes.
A smaller proportion of lighter colored eyes have pupils of normal or nearly normal size but lack a part or all of the dark iris pigment. Occasionally a
segment of iris is unpigmented. Out of 266 eyes of this strain examined in mice of the sixth and seventh inbred (brother-sister) generations, 120 had
the iris reduced (98) or missing (22); 48 without gross iris defects had pigment reduced (34) or missing (14); while the remaining 98 had no gross
defects. Of the 133 animals examined, both irides were normal in only 28, so that in this strain the iris defect expresses itself in about
80 per cent of the individuals and in some 63 per cent of the eyes. Thus there is a high degree
of asymmetry in gross expression. The right and left eyes have about the same probability of being abnormal as judged by
ophthalmoscopic examination. The proportion of eyes with histological abnormalities is
unknown. The differences in iris character between two sides of the same individual and between individuals of this inbred strain are apparently
non-genetic.
In contrast with this strain is another of similar derivation. This strain (Anophthalmic), now in its 22nd brother-sister generation, has a genotype
which is similar to that of the Aniridia strain except that it lacks the piebald gene (atat BB ruru sisi). It contains a gene or genes which express
themselves in some 65 per cent of the individuals, often asymmetrically, by causing a defect of the bulbus and/or cornea which varies from bilateral
anophthalmia to unilateral staphyloma. In none of the eyes of this strain with normal corneae have the iris defects of the Aniridia strain been seen.
Tests of Inheritance of Anisocoria.-The most direct test of the hypothesis that anisocoria is an effect of the piebald gene is to cross the above two
strains which differ primarily at the piebald locus. From this cross 26 F, animals were examined. All were non-piebald with ruby eyes and normal
irides.
Fourteen of these Fi's were backcrossed to the spotted ruby (An) strain
and produced the results shown below:
NORMAL IRIDES ABNORMAL IRIDSB TOTAL
Non-piebald 116 4 120
Piebald 72 43 115
Of the non-piebald offspring only 3.3 per cent had any abnormality of the iris, and of these one had an oval pupil on one side, and one had a
slightly enlarged pupil on one side. Of the other two, one had no detectable iris on one side; the other had an unquestionably enlarged pupil on
one side. These last two cases are exceptions to the rule that anisocoria appears only in piebald animals. They may of course have been homozygous
for the piebald gene ss which for some reason did not express itself in spotting, but there is no experimental test of this possibility. Two cases
of what then was called heterochromia iridis were noted among 48 nonpiebald ruby eyed mice.' Although these were not examined with the
ophthalmoscope, they were probably exceptions of the sort noted above.
The significant feature of the results was the appearance of iris abnormalities in some 37 per cent of the piebald animals. This is not far from
the frequency of iris abnormalities in the An piebald strain at the time (F3) when- the cross was made (15 out of 32 animals examined showed
gross abnormalities of the iris).
The data show that piebald spotting and iris defects are closely associated
and do not exclude the hypothesis that they are due to the same
genotypic cause.
Anisocoria in Other Spotted Strains.-If the iris defects are due to the
VOL. 38, 1952 873
GENETICS: DUNN AND MOHR
action of spotting genes they should be found in other spotted stocks unrelated to the above. Among 39 piebald mice from an inbred black Brachy
line in this laboratory known to be genetically ss, 5 cases of small or absent iris were found. Among 63 piebald animals of Line 3 (Black agouti) in
this laboratory, eleven had gross defects of the iris. One case was found among 95 animals with spotted face in Dr. Schneider's colony at Rockefeller
Institute for Medical Research and one case among 15 animals with the "splotch" mutant condition in strain C57 black. The last two strains
are not known to carry the piebald gene. These 18 cases were all found in black-eyed animals so that although the ruby-eye phenotype makes
the iris abnormality easier to identify with the naked eye, ruby is not a necessary condition for its development. In all of the 102 black-eyed piebald
animals from the Brachy line and Line 3 a deficiency of the chorioidal pigmentation could be recognized ophthalmoscopically. This confirms
earlier findings of defective chorioidal pigmentation in piebald mice.2'3 In ruby animals this deficiency is difficult to recognize ophthalmoscopically,
but histological examination indicates that it is a general feature of the piebald phenotype in these animals too.
Test crosses between a piebald with abnormal iris from Line 3 and one from strain An produced 5 spotted animals with abnormal iris and 9 spotted
without iris abnormality, showing a similar genetical basis for anisocoria in those two unrelated strains. A test of one anisocoric animal from
the Brachy strain by strain An was negative yielding only 15 animals with normal irides. Because of the small numbers involved, this test was not
conclusive for the question at issue.
Among the 176 self-colored animals from a variety of strains, examined with the ophthalmoscope, not a single case of anisocoria was found. However,
15 cases of abnormal iris were found among 100 albinos examined from various strains. Six of these cases, all from an inbred albino Fused
strain of this laboratory, had coloboma iridis and were thus quite different from the anisocoria associated with piebald. The Fused strain was tested
and found-not to carry piebald. These and other cases of eye abnormalities found in several strains will provide good material for ophthalmological
study.
Finally, a new occurrence of Aniridia in several related animals from a mixed stock has been observed. In the first case, a brown piebald animal,
both eyes were light pink. When tested by ruru SS this animal was shown to have the dominant allele of ruby (Ru), giving 26 non-ruby in F1, and
matings among F1 animals according to birth classification showed segregation, for uncolored irides (80 colored, 29 uncolored) for one eye
color mutant only, viz., ruby eye.
In F2 and backcross progenies, ophthalmoscopic examination of irides at weaning revealed several cases of anisocoria including aniridia, but these
874 PROC. N. A. S.
GENETICS: DUNN AND MOHR
occurred only among spotted animals. Since the case was apparently similar to that analyzed above it was not studied further. It does show,
however, that although anisocoria is easier to detect due to the pinkness of the eye, when it occurs in ruby-eyed animals, it may exert its full effect in
spotted animals without the ruby mutation and thus simulate a mutation.
Discussion.-It is evident from the above that the type of related iris
abnormalities first noted in strain An and shown there to be associated
with the piebald condition are found also in other unrelated piebald strains
of various genotypes and rarely in spotted strains not known to carry the
gene s.
This is probably not the first time that eye defects have been noted in piebald mice, although it is the first time that their genetic nature has been
analyzed. Durham2 in 1908 noted "ruby eyes" in three mice. In each case the choroid pigment was greatly reduced, and in each case the abnormal
individual was chocolate (brown) piebald. No information on size of pupil was given. Later Gates3 examined a population segregating for
piebald, agouti, black-brown, and density and dilution, and found that all spotted animals and only spotted animals lacked chorioidal pigmentation.
External defects, such as pinkness or heterochromia, were not noted. The present study indicates also that in piebald mice the chorioidal pigmentation
is generally defective.
There is thus a general association between spotting and eye pigmentation
which suggests the following working hypothesis: The genotype responsible
for piebald spotting (and perhaps for certain other spotting types)
produces its effect by increasing the probability of death or failure in function
of the pigment cells (melanophores). In the mouse these cells are derived
from the neural crest (Rawles4) and reach their destinations by migration.
The normal development of the iris depends upon actual or inductive
contributions by such cells. If their probability of survival is lowered, the
failure of pigment development in local areas of the skin will be accompanied
by failure to invade the choroid, and an increased probability of
failure of development of the stroma of the iris or of its pigment or of both.
In some such manner as this the effect of a "spotting gene" upon a structural
component such as the iris may be envisaged.
Since both amiridia and spotting occur as genetic variations in man, a
study of their relations in human families may yield some evidence on the
hypothesis suggested.
* Visiting Fellow of the Rockefeller Foundation, 1949; now at Genetics Institute,
The University, Oslo, Norway.
I Dunn, L. C., PROC. NATL. ACAD. Sci., 31, 343-346 (1945).
2 Durham, F. M., Reports to the Evolution Committee, Royal Society, 4, 41-57
(1908).
3 Gates, W. H., Carnegie Institution of Washington, 337, 83-138 (1926).
4 Rawles, M. E., Physiol. Zool., 20, 248-266 (1947).


Heterochromia (Eyes of different colors)

Eye pigmentation is determined by melanocytes. The presence of melanocytes in the iris is dependent on innervation by sympathetic nerves, which grow out of the spinal cord (anterior roots of the first and second thoracic segments) and follow the carotid artery to the head. Thus, one way to develop heterochromia is to have an injury to this portion of the nervous system. Congenital cases are generally inherited as autosomal dominant traits.

Heterochromia is usually (and maybe always) associated with Horner syndrome, Waardenburg syndrome, or piebald syndrome.

Piebald trait: "The features are white forelock and absence of pigmentation of the medial portion of the forehead, eyebrows and chin and of the ventral chest, abdomen, and extremities. The borders of unpigmented areas are hyperpigmented. Heterochromia iridis occurs in some." (from OMIM 172800) A white forelock is a common symptom. Deafness is usually not associated with this trait. Piebald is common among mammalian species, and probably involves a defect in the migration of the neural crest cells (which include the melanocytes), but the exact mechanism is not clear. The homologous gene in mice is W, dominant white spotting. A similar gene has also been studied in pigs. The trait is dominant in humans: homozygotes are rare and are completely lacking in pigment in hair and eyes. The gene involved is the KIT protooncogene, whose viral homologue is in feline sarcoma virus. The cellular gene encodes a transmembrane tyrosine kinase that is a receptor for stem cell factor (SCFR), which is necesary for normal melanogenesis, as well as blood cell formation (hematopoiesis) and germ cell differentiation (gametogenesis).

This is a BROWN/TAN MERLE

Browns have self-coloured BROWN noses

There is ABSOLUTELY NO BLACK on a Brown Merle

(*Some registries label these as Chocolate)

This is a BLACK/TAN MERLE

Black/Tan Merles have BLACK noses

This is of the SOLID body color
This is a RED MERLE

RED Merles can have black or brown noses

(*Reds can have BROWN noses but Browns can NOT have Black noses)

 

This is a BLUE/TAN MERLE

Blue Merles have BLUE noses

This is ABSOLUTELY NO black on a blue merle
This is a FAWN/TAN MERLE

Fawn Merles have Brown noses

This is ABSOLUTELY NO black on a fawn merle

We need to keep in mind that a BLUE Merle in Rat Terriers is not the same as a BLUE Merle that is commonly known in the Australian Shepherd. Australian Shepherds do not have as many colors as are associated with the Rat Terrier therefore they do not have to worry about the TRUE genetic code associated with them.
BLUES in Rat Terriers have ABSOLUTELY NO black on them.


While MERLE Rat Terriers may be few and far between they are in no way "RARE".
I would describe them as "UNIQUE".
A Merle is easily reproduced. All it takes is for one parent to be merle to produce on the average of 50% Merle patterned offspring.
NOW to produce the dilute colored ones requires either a lot of luck or knowledge of genetics and how to apply it to your breeding program to produce the colors that you would like.

Just in case some of you do not know the difference and are confused, a color and pattern are two totally different things. Color is the base color of a dog. Pattern is the form/shape/placement of which the different colors take place. Patterns are DISTINCTIVE visual appearances.







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