The APBT standard and the recent emergence of the Merle pattern in the breed.

By Scot E. Dowd Ph.D.

The recent appearance of merle patterned APBTs and a couple breeders specializing in the "new" and "rare" color pattern has stirred up a controversy in the APBT community.  The general thought among those that have been around the breed for the greatest number of years is that, these new color patterns were brought about by unscrupulous outcrossing to a separate breed such as Catahoula Leopard dogs.  Several breeds are known to carry merle as a color pattern but the APBT is not one known to have  ever carried this "infected" allele.  What is known is that Catahoula Leopard dogs along with pit bulls are often used in the sport of hog catching and it is known that crosses of these breeds have been made in attempts to produce more competitive catch dogs.

The following c omments from the APBT standard committee provide prelude to a brief essay on the merle locus in relation to the APBT

 Walt Pasko   "I feel the emergence of the merle color pattern in our breed has raised the questions of how it was introduced into our breed and what health problems the merle gene could cause. From all information I've read, I have to recommend that the merle color be made a disqualification in the APBT Breed Standard." 

Carol Gaines Stephens "I strongly oppose the color pattern 'merle' in the APBT since it has never been there in the past and has just recently risen it's ugly head with the popularity of the catch dogs in the south.  I have spoken to several people from the south that say that they know and do so themselves, cross the APBT with the Louisiana Catahoula Leopard Dog to make a better catch dog.  If the gene has never been present in all these decades/centuries then how did it finally come about just recently?  I am a firm believer in leaving the standard the way it was originally, but when something surfaces that has no rhyme nor reason, then I think we have to address the matter."

Cheryl Larum "I am in agreement with the other committee members on the merle issue"

Scot E. Dowd  " First it should be noted that there are ways that the merle can remain hidden such as within a complete phaeomelanic coat where the merle would not be evident, also there are cryptic merles, however this absolutely would fail to explain the relatively recent appearance of this color pattern in the APBT.  I feel that another allele with defined health problems associated with this locus, is not a positive thing for our breed"

The following information is submitted on behalf of the NAPBTA standard committee - Scot E. Dowd 

There are two issues of concern with the merle as a color pattern.  The first, as mentioned, is that merle pattern in the APBT may have come about through unethical outcrossing to another breed of dog.  This practice would then have been followed by falsely registering such a outbred animal either with the ADBA or UKC as a purebred APBT.  Such false registration would be termed hanging papers.  The other issue is related to the health aspects of the Merle allele.  Here I will try to answer the predominant questions that arise regarding the merle allele and the APBT without making a judgment of my own other than that expressed above.

Why is a color or color pattern so important to the stewards of the breed? 

The entire process of coloration and color patterns in dogs starts with embryonic development.  The specific cells that become melanocytes (pigment producing cells) are derived entirely from the neuronal crest of the embryo.  This essentially means that pigment cells are directly produced along with the same cells that give rise to the nervous system.  Though not entirely true, it can be assumed that if you have defects in genes associated with color genetics you might also have nervous system defects because both types of cells are derived from the neuronal crest.  This provides a logical genetic indicator and explains why it is likely that certain dilute or patterned dogs, such as extreme piebalds, or other types of homozygous dilutes common in the APBT, as well as those that may be carrying the Merle pattern are prone to psychological, neurological and/or immunological problems found in other breeds that carry these alleles.

What is merle?

Merle like other dilution alleles acts to lighten whatever color would otherwise have been expressed.  However, with merle the lightening effect is not spread evenly over the coat, but produces patches of undiluted color (dappled pattern) scattered over the dog's body.  The merle gene when heterozygous Mm (only one copy of the gene) on an otherwise black dog produces a blue merle which is phenotypically a bluish gray dog that is dappled with full color black spots.  A homozygous or MM dog (carrying two copies of the merle gene), often called a double merle or a homozygous merle, will be a mostly white dog (similar to an extreme Piebald). The normal state of the merle locus is dual recessive mm and completely lacks the offending transposon resulting in normal color. 

Maybe merle has been in the breed throughout its history and only now is it being noticed?  

The response to this question is also genetic in nature.  The genetic and phenotypic nature of the Merle locus and the merle allele (M) is such that it would not remain unnoticed in a breed and suddenly appear.  It would take crossing to another breed that carries the merle allele for it to be transferred into the breed. The reason it could not remain invisible or hidden is because the Merle allele is expressed with incomplete dominance.  This means, if it is within the genome at all, even in a heterozygote (one copy of the gene) state, it is still expressed and evident.  The M allele is not found in all breeds; in fact most breeds do not carry it.  Finally, this specific transposon cannot arise spontaneously or through mutation as some have claimed.

What are the health problems associated with the merle allele?

The merle allele like a couple other dilution factors when expressed in a homozygous state is correlated to psychological, neurological, and usually immunological issues.  Here I will mention a few of the issues.  The first are eye development problems that are superficial in nature affecting appearance such as heterochromia iridis (A difference of color between the iris of one eye and the other), thus a dog with one brown and one blue eye has heterochromia iridis.  Note that this defect is not necessarily or always indicative of having the merle gene because it can also be found in dogs with extreme piebald or double blue dilution for example.   In addition to superficial indicators there are also major effects such as absence of tapetum lucidum.  Tapetum lucidum is a reflective substance that lines the back of the dogs eyes.  This reflective structure acts like a mirror and reflects light back through the retina, like a satellite dish giving the retina two chances to catch the light.  Dogs that lack tapetum licidum have night blindness or reduced ability to see in low light.  Another defect is lack of retinal pigment and microphthalmia. Microopthalmia (smaller than normal eye) is described as dogs having prominent third eyelids and seemingly small eyes which appear recessed in the eye socket (enophthalmos).  Another problem known as coloboma is actually a physical cleft in a portion of the eye, particularly the iris .  In addition to the eyes which are a key indicator of neurological defects, there is also evidence for effects on the ears that result in reduction in auditory sensitivity or complete deafness because the merle color locus exerts epistatic effects on ear development.  Excessive white or dilution in a dog of any color can be a warning sign of potential hearing problems. If there is no pigment in the inner ear the dog will be deaf; white ears are more likely to lack inner ear pigment.

More technically, what is the genetic explanation of the merle pattern?

The merle allele is considered to be caused by a transposon or transposable element.  A transposon is a piece of DNA that has the potential to actually jump out of, or excise from the gene it has infected (disrupted), during cellular division and genetic DNA replication. This means that while melanocytes are migrating from the neuronal crest during embryonic development the merle transposon can remove itself from the gene in some of the melanocytes when they are derived and produce normal coloration on those parts of the coat to which they migrate.  Thus, the merle allele acts to cause eumelanic areas in the coat, to become diluted, but other areas to be fully and intensely pigmented.  Such fully colored areas occur in scattered patches throughout the body.   The merle locus is autosomal (not carried on one of the sex chromosomes) acting as a dominant mutation (it is expressed in all dogs that carry this gene).  It should also be noted that genetically such transposons do not arise spontaneously but must be passed from sire and/or dam to offspring.  This means that if the APBT did not carry this allele to begin with, then only through outcrossing to another breed, that does carry this transposon, could it be integrated into the APBT genome. 

  Black               As-D-E-
  Blue                 As-ddE-
    Black & Tan   atatD-E-
Red                AyD-E-
Fawn              AyddE-
Brindle               Ay-D-Ebr-
     Blue Brindle         Ay-B-ddEbr-

CONCLUSION:

Question # 1: In times past, the blue color was considered as rare in the breed.  How are kennels now producing blue dogs in such great numbers?
 

The D Locus pair is the loci that modifies the dark Pigment pattern to blue with the homozygous recessive (dd) alleles. Because kennels are selecting blue individuals which are homozygous recessive (dd) the only allele that a blue parent can give to their offspring is the recessive (b) allele. Breeding two blues together doubles up the recessive trait to (dd) which modifies the dominant (As) allele to blue. If a resulting offspring happens to get the (Ay) allele from one of its parents, then the (dd) will modify this color to a fawn/bluies.  If breeders are breeding dogs for the blue color, since the gene is homozygous recessive, 100% of their offspring will be (dd) and depending on the (As) or (Ay) allele on the Locus A series will have blue or fawn/bluies pups.

Question # 2: It is possible to produce a puppy with a black nose from two parents, both with red noses?

The red nose in our breed is produced from the Locus B pair with the homozygous recessive (bb) genotype in combination with the (Ay-) genotype. Mating two dogs with the genotype (bb) the result would be 100% of the pups having red noses (bb). If a pup with a black nose was produced in one of my litters where both parents had red noses, then DNA testing with parentage verification would be necessary to determine the actual sire to the black nose pup.  New alleles can be produced through mutation, but proof of parentage would have to be determined using DNA to rule out a duel sired litter.

Question # 3: Where does the chocolate color come from?

The chocolate color comes from the Locus B pair with the homozyous recessive (bb) in combination with the (As) allele. The dilution gene (dd) will also modify this dark chocolate to a light or almost milk chocolate. In most cases, the nose color is also light brown or as we say 'chocolate/red nose'.

Question # 4: How did I produce a brindle from a line that has never had brindle dogs?

The Locus E Series (Ebr) allele causes the brindle color pattern when this dominant allele is present  in combination with the (Ay) allele. The (Ebr) allele is dominant except in dogs with the (As) allele. In the case of the (As), the (Ebr) allele does not have a light pigment to work on, the brindle will remain hidden. In tracing a brindle dogs bloodline, somewhere in the generations you should see a brindle dog since the (Ebr) allele is dominant. Again, if this was my litter, I would ask for  parentage verification using DNA to rule out a dual sired litter.