The Color Wars
Understanding the genetics behind controversial colors in the Tibetan Mastiff
Perhaps, with the growing popularity of the Tibetan Mastiff, the time is finally ripe to try to understand the inheritance of coat color in the beast. This is especially important if we want to understand what colors are present in the Tibetan Mastiff’s native country and the combinations produced when those genes and colors are recombined.
Coat color in all mammals is rather similar in that the same genes are involved. However, different mutations of those genes are found in different species and different breeds of dogs. This is not simple, so bear with me. I will discuss four or five different genes and their mutations that affect the coat color in Tibetan Mastiffs.
There are others, but they are either not well understood or have no mutations that affect coat color in Tibetan Mastiffs, although there may be mutations of those genes that affect other breeds of dogs or mice. I will try to tell you when we are fairly certain how things work and some of the evidence why we believe that, and I will also tell you when there is either conflicting evidence or we are just not certain.
First, let us look at the known colors in Tibetan Mastiffs so we understand what we are trying to explain. There is solid evidence in pictures and live dogs for all the following colors: black (black all over, possible white spots on chest, which can happen with any color); gold (ranging from very pale cream color to dark red and with varying degrees of black tipping on the hair); black and tan; blue (gray); blue (gray) and tan; chocolate (brown); chocolate (brown) and tan, and some combinations of gold and dilute (more later).
Before we examine the genes that produce these colors, we need to do a very superficial review of genetics. All dogs have two complete sets of chromosomes; hence they have exactly two copies of every gene. Those two copies can be exactly the same (we call these animals “homozygous” for that gene) or the two copies can be different (we call these “heterozygous” for that gene). In order for any gene to be studied as to its effect on coat color, that gene must have mutated some time in the past so that there are two different forms of the gene present in the breed we are studying; we call these different forms of the same gene different alleles.
There may be one or several different alleles for any gene, but any given dog can only have two of them because they only have two sets of chromosomes where the genes reside. Often one allele is dominant (written uppercase, A) and others recessive (written lowercase, a). Sometimes there is a dominance series where more than two alleles are known at the same locus and each is dominant to all the ones below them in the hierarchy. As we know, any dog gets one set of chromosomes (genes) from its father and one set from its mother. So no dog can have more than two copies of any given set of alleles (that is, more than two copies of a single gene).
The color of most Tibetan Mastiffs is determined by whatever alleles they have at the A locus (another term for gene, just specifying a place on the chromosome). There are many different alleles (different mutations) of this gene known in dogs and other mammals. However, I now think only two are found in Tibetan Mastiffs (this is open to debate and I will explain my thinking further on). The two alleles that are certain are A (formerly Ay), producing gold, and at, producing black and tan. Notice we have written A for gold because it is dominant, and small at for black and tan because it is recessive, and a dog has to get two copies for it to be a black and tan.
The most difficult color to discuss genetically is the solid black. There are at least two different genes in Tibetan Mastiffs that can produce a superficially black dog. One of those is a dominant gene (only one copy of a dominant gene is necessary to produce an effect) and the other is recessive (both copies of the gene have to be the recessive mutant in order to produce a black dog). The dominant black gene was once thought to be an allele of the A locus (gene), but now is considered to be a different dominant gene at the K locus. This means that if a dog gets a single copy of dominant K it will be black, so any dog that is not black must have two copies of the recessive k. This gene was common in some of the first dogs to come into this country, but, because they were of mediocre type, that line has not been perpetuated. I don’t believe the dominant black is very common in the breed at this time in the United States.
The other way to get a black dog is by way of a recessive black gene, which I am going to call bk. So to get a black dog this way, the dog must have two copies of the recessive bk, whereas one copy of the dominant allele Bk will produce a dog that is some color other than black. These recessive black dogs appear superficially black, but usually have a few brown hairs between the toes. ( See Picture #1) Apparently in some breeds there is a recessive black mutant at the A locus; however, this mutant has not been found in Tibetan Mastiffs. The recessive black gene that we have behaves genetically as if it is not an allele of A. (The evidence for this is that I bred two gold Tibetan Mastiffs and got gold, black and tan, and black from the same litter. This cannot happen if they are all alleles of A.)
Now the colors that are causing the most controversy are the ones that are left. Any Tibetan Mastiff that does not have the dominant K, or two copies of recessive bk, or one of the dilute genes (to be discussed later) will be some shade of gold (dog has one copy of dominant A) or black and tan (dog has two copies of recessive at). This gold color can range from very pale cream to very dark red. (See pictures 2-8) There can be darker sabling on the coat ranging from almost undetectable amounts in the cape on the neck to extremely heavy sabling that darkens the entire dog. All of these dogs are carrying the same A gene for gold. Both the range in the gold and the degree of sabling are due to different modifying genes where multiple genes contribute to the effect in some additive way. We know this because when we look at the shade of gold or the degree of sabling in a large number of dogs, these dogs can be arranged in a continuous spectrum from light to dark gold and from little to extreme sabling.
Despite the fact that these two effects —darkening of the gold color and dark sabling on the coat – both appear to be controlled by multiple genes working additively, they are not the same genes in the two cases. If it were the same genes then light gold color would be found in the same dogs with little sabling and dark gold would be found in the same dogs with heavy sabling. Such is not the case; there is no correlation between darkness of the gold and degree of sabling in the pattern. If it were a single gene controlling either one of these effects, you would have only two or three discreet categories, and that is not the case. The degree to which both effects are present in dogs is completely continuous.
Very light gold.
Very dark gold.
Very Dark Red.
Furthermore – and this is an important point in the color wars – whatever genes control the range of colors in the gold dogs are exactly the same genes that control the gold regions (tan points) on black and tan dogs. (See Pictures 9-12) Notice that the range of color on the tan points parallels the range of color on the gold dogs. The proof that the same genes are controlling both the gold and the tan on the black and tans comes from observations of crosses. When you do crosses expecting both golds and black and tans, the tan (gold) colors on the two types will always be within the same range. You won’t normally get dark-gold dogs from crosses where the black and tans in the litter are all very light black and creams.
Now we can understand why the proposed new AKC standard for the Tibetan Mastiff is self-defeating. Medium-gold dogs are allowed, but very light gold dogs are disqualified. On the other hand, black and very light cream dogs are perfectly acceptable. If a black and light cream dog is bred with a medium-gold dog, about one quarter of the offspring will be light enough to be disqualified, even though both parents were an acceptable color. In other words, the proposed standard is trying to eliminate modifying genes in individuals that are gold (A, a) or (A,A), but allow those same modifying genes when they are present in a black and tan dog (at,at). This is elimination of good breeding stock in colors that are found worldwide is ridiculous in an age when the advantages of maintaining genetic diversity are apparent to everyone.
Black and very light cream.
Black and light tan.
Black and medium tan.
Black and red.
There are two dilute genes known in Tibetan Mastiffs. These genes affect any area of the dog that would be covered by black pigment. The first of these is the d locus. The dominant allele D is necessary to have a black dog or a black and tan dog. The recessive in homozygous condition (d,d) produces a dog that is blue (gray) in any dog that would otherwise be black (that is, it is either K- or it is bk,bk at the two genes producing black). It will also have a gray nose and somewhat lighter eyes and eye rims, as all of these areas depend upon black pigment. Dogs that would be black and tan (a,a) dogs that also get two copies of the recessive d will be blue and tan.
The second dilute gene in Tibetan Mastiffs is brown, also called chocolate and designated b. The dominant gene B is necessary for normal black pigment. In the presence of the homozygous recessive genes (b,b), the black areas will be brown or chocolate. The nose and eye rims will also be lighter. Just like the situation with blue, dogs that would have been black and tan (a,a) will be chocolate and tan if they also get the recessive b,b.
All dilute dogs have nose leather some color other than black. Very light gold dogs are not a dilute of any kind because both the nose leather and eye rims are black.
The proposed new AKC standard for the Tibetan Mastiff also accepts blue dogs but discriminates against chocolate dogs. There is actually more old evidence in support of chocolate as an allowed color than there is for blue. There is a very old Chinese painting of a Tibetan Mastiff that was given as a gift to an important politico that is very clearly a chocolate and tan dog. Even the color of the toenails in the painting is accurately lighter than those on a dark-gold dog. On the other hand, I have seen no old quotes about blue dogs noted in Tibet or China.
Apparently, one of the first Tibetan Mastiffs imported into Europe was carrying the blue gene but none got the chocolate gene. This is not surprising, since both are apparently rare and the sample size in the imported dogs was so small that it was not representative of the whole population. These are called founder effects in genetics, but now that we have a better idea what the total Himalayan population looks like (with all the pictures and recent visits to China), we should not be basing our standard on outdated data from the offspring of a very small and skewed sample (those dogs that were imported into Europe). Dogs with the genes and colors discussed here comprise the vast majority of Tibetan Mastiffs in the world, and all of these colors should be equally acceptable to the standard.
About the Author
Most of Charles Radcliffe’s working career has been spent in zoos and universities, studying feeding behavior in venomous snakes. He first learned of Tibetan Mastiffs in the 1960s as a graduate student at Indiana University, but none were in the country at that time. He finally got his first Tibetan Mastiff in 1985, and has since produced 28 litters and many champions. Among these, the most well known was “Barnes” (Timberline’s Barni Drakyi), the first Tibetan Mastiff to win the breed at Westminster, co-owned with Rick Eichhorn and Lois Claus. He is pictured here with “Zora,” who, he says, “is a wonderful guardian and companion and the dam of two great litters.”
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Mon, 07/26/2010 - 1:00am