Coat Color Genetics
in Tibetan Mastiffs
© Charles W. Radcliffe & Matthew J.
Taylor 2008
"Mere color, unspoiled by meaning, and unallied
with definite form,
can speak to the soul in a thousand different ways."
Author: Oscar Wilde
Have you ever been to a show that sports Tibetan Mastiffs appearing
in a variety of colors and asked yourself where these colors come
from, or how a breeder can predict what colors will appear in a
given litter? Well, hopefully, this article and some color photos
will begin to shed some light on this colorful subject. Before
we get to the fun stuff, however, we will have to review a little
genetics.
Most
vertebrates, including dogs, have two copies of every chromosome
(except sex chromosomes). Since genes make up the chromosomes,
this means that there are two copies of every gene present in every
cell of every dog. However, the two copies do not have to be identical.
They can be different versions (alleles) of the same gene. For
example, one could be an allele for Black coat and one for Gold
coat. Hmmm, this must be a Black and Gold spotted dog! No, because
some alleles are dominant to others. Since Black is dominant to
Gold, this dog will appear as solid Black.
Although the above example shows a simple relationship between
two alleles located at one gene locus, coat color in dogs is ultimately
quite complicated and not fully understood. Many genes, some of
which have more than two allele choices, control the variety of
colors and patterns seen in canine coat color. The many different
genes interact by intricate rules, to create the final coat.
For
the sake of discussion, each gene locus and all the alleles related
to coat color are given alphabetical designations. Dominant alleles
are shown in capital letters and recessive alleles are given in
lower case letters. For the purposes of this article, only those
loci and alleles seen in the Tibetan Mastiff will be discussed.
Given that this breed is so colorful, that happens to be most of
them.
Okay, back to our example. The gene locus involved in the example
is the Agouti, or A locus.
Three alleles of this gene are found in the Tibetan Mastiff: the
allele for Black, designated A ; the allele
for Gold Ay ; and the allele for Black
and Tan, designated at. These alleles
are given here in order of dominance, where Black is the most dominant
and Black and Tan is the most recessive. Although there are three
different alleles that are found in the Tibetan Mastiff breed,
any given dog can only carry one or two of the three; two copies
of the same allele, or two different alleles. (There is some new
evidence that the dominant black gene may be at another locus,
thus not an allele in this series. If so this would change some
of the expectations from any breeding with a dog carrying dominant
black. The relationships between Ay and at would
be unaffected. However, the presence of the dominant black gene,
now given another name at a new locus would mask any differences
at the Ay/at locus and just
make the dogs look black.)
Why
two? Well, every dog has two sets of chromosomes (they get one
set from each parent) and each set of chromosomes contains one
copy of each gene. Hence every dog has two copies of each gene,
one from each parent. In our example, the dog is A/Ay .
This is written so that we see both alleles, Black and Gold, with
the most dominant allele, Black, written first. Remember, this
dog appears Black, but so would a dog that is A/A or A/at .
Thus, if a dog appears Black (image 1), we may have no idea what
the second allele is until we breed that dog and find out what
is passed to the offspring. The same is true for dogs that are Ay/Ay or Ay/at as
they will appear Gold (image 3), because Gold is dominant to Black
and Tan. Only when a dog is at/at does
it appear Black and Tan (image 6).
What we call "Gold" actually ranges in color from Light Gold (image
2) to rich Reddish Gold (image 4), with the precise shade being
determined by modifying genes. We
do not know the identity or the mode of action of these modifying
genes, but we can describe a model of how they are inherited based
upon what we see when we breed the dogs. They act like, well, a
collection of checkers (lets say red and white checkers). For example,
if a dog has mostly red checkers in its collection, it will appear
Reddish Gold. If a dog has mostly white checkers, it will appear
Light Gold, and if a dog has a balance of red and white checkers,
it will appear Gold. Each puppy receives a handful of checkers
from each parent, so Red Gold puppies are unlikely to come from
two Light Gold parents and vice versa. These same genes work to
determine the shade of tan on Black and Tan dogs. The dogs shown
in images 5, 6, and 7, are all Black and Tan (determined by the A locus),
but the tan varies considerably. As in the example above, Black
and Red-Tan puppies are unlikely to come from parents that are
Light Gold and/or Black and -Tan (Light Gold Tan).
So
far the discussion has focused solely on dogs that have what is
called full pigmentation. That is, their genetic makeup allows
them to develop all colors to their fullest extent: pitch Blacks
and bright Red-Golds, Golds, and light Golds, all with black noses.
There are some genes whose effect is to dilute the pigmentation.
In other words, whatever A locus
alleles the dog has will be expressed as expected, but the pigmentation
will be diluted, even in the eyes and on the nose leather. In the
Tibetan Mastiff, two different genes cause dilution effects. Each
of these genes has only a dominant and one recessive allele.
The first of these genes is the Dilute, or D locus.
The dominant allele D is necessary for
full pigmentation. The effect of the recessive allele d when homozygous
(two identical alleles in the same dog), is to dilute Black coat
and nose and eye pigment toward Blue/Grey. Although this gene is
completely separate from the A locus,
there is an interaction that produces the final color of the dog.
So, if a dog is A/-, d/d (where the dash
represents any allele choice) it will appear Blue/Grey (image 8).
If a dog is at/at, d/d then it will appear
Blue/Gray and Tan (image 9).
The
second gene that serves to dilute coat and pigment is the Brown,
or B locus. This gene works the same
way as the D locus. That is, when both
copies of the B gene are recessive ( b ),
in this case the Black coloration fades to Chocolate/Brown. Again,
this gene is completely separate from the A locus,
but the interaction of the two loci determines the final color
of the dog. Thus, if a dog is A/-, b/b it
will appear Chocolate/Brown (image 10). If a dog is at/at,
b/b it will appear Chocolate/Brown and Tan (image 11). Again,
one dominant allele B is necessary in
any dog to get the normal black pigment. Any dog with black nose
leather is not homozygous for any dilute gene. At least one dominant
allele of both of these genes is required for the full black nose
coloration.
Finally,
a dog can receive a pair of recessive alleles at both the D and B loci.
Although, for the moment, and to our best knowledge, the Western
world has not produced a Tibetan Mastiff with this genetic makeup,
it is thought that one would appear somewhat like a Weimaraner
in color. We are currently referring to this color as Double Dilute.
Of course, this would dilute the black coat and pigment in Black
and Tan dogs as well, so there could be dogs that can appear as
Double Dilute and Tan (at/at, b/b, d/d).
Gold
dogs with dilution are a little more complicated. They still appear
gold, but appear more washed-out than normal, and their nose leather
may appear Blue/Grey, Chocolate/Brown, or some muddy combination
of the two. These dogs are Ay/-, d/d (where
the dash represents either Ay or at ), Ay/-,
b/b , or Ay/-, b/b, d/d . The authors
are referring to ALL of these colors as Gold Dilute (image 12),
as determining exactly which dilutions are at work may be difficult.
All of the dogs that are homozygous for either or both of the dilute
genes will have nose leather that is obviously lighter than the
normal black color. It may be tan or light grey or some other combination
of the two.
So far, so good, and the above mechanisms are well established
in the dog world, but what follows is not. There is one type of
Black Tibetan Mastiff that is turning up more frequently in American
litters. Breedings of two Black and Tan dogs ( at/at mated
with at/at ) are yielding puppies that
are all Black. This is completely unexpec
ted! Since
the allele for Black, A , is dominant
to Black and Tan, at , neither of the
parents could be carrying Black A , or that dog would appear Black
itself. So, some other explanation must exist for these dogs to
appear solid Black. Breeders have found that when bred, some of
these dogs produce as if they were Black and Tan, not as Black.
So, genetically, they are just what one would expect, at/at .
One explanation for the conundrum would be that there is a recessive
modifying gene present that completely masks the tan, yielding
a Black dog. In some cases, a few tan hairs develop between the
toes or under the tail, and over time may become more traditional
in patterning. However, at birth, these Black pups are indistinguishable
from the true Black colored puppies (A/-) .
We are now referring to these as recessive blacks. A recessive
Black female is seen in image 13. It is also clear that this recessive
masking gene can mask all the gold on genetically gold TM as well.
The only way to tell whether a recessive black TM is really a masked
black and tan or a masked gold is to breed it and see what you
get. These recessive black dogs can also appear in matings of Gold
dogs and the explanation is exactly the same. Al though we do not
know of any cases, we assume this masking gene would also affect
dogs carrying the two dilute genes for blue and chocolate, thus
converting genetically blue and tan and chocolate and tan dogs
into blues and chocolates respectively. Breeding tests have conclusively
demonstrated that this recessive black gene is not an allele in
the Ay,at series.
For
clarity, and to move us to the fun part, a couple of genetic word
definitions will help; the first is phenotype. This refers to what
is seen. A Tibetan Mastiff's color phenotype may be Black, for
example, or Black and Tan. The second word is genotype, which describes
the genetic recipe (our alphabet soup) carried by that dog. For
example, a Tibetan Mastiff genotype may be A/at (which
may appear as a Black phenotype), or at/at (which
may appear as a Black and Tan phenotype).
Oh yeah, here is the fun part. Ready?
Let's
look at an illustration using dogs that are phenotypically Black
(not including the recessive blacks), that is , these are Black
dogs with no tan points, although they may have markings like white
on the chest (to be discussed in a future article). Keep in mind
that Black dogs must have at least one copy of the dominant allele A .
Now for this dog to be full-pigmented Black, it must also have
at least one dominant allele D and at
least one dominant allele B . If we do
not know any more than how the dog appears, the genotype of this
dog can then be written, A/-, B/-, D/- (as
alleles represented by the dashes cannot change the phenotype --
how the dog appears). So, we cannot tell by looking at the dog
whether it is carrying any or all of the recessive alleles. Only
breeding will reveal whether this dog is carrying the Gold allele
( Ay ), the Black and Tan allele ( at ),
the recessive Blue/Grey allele ( d ),
or the recessive Chocolate/Brown allele ( b ).
So, how does a breeder find out the exact genotypes of his/her
breeding stock?
The
first thing to remember is that each parent contributes to their
offspring exactly one of the two alleles it is carrying for every
gene in its chromosomes. This means that when the sperm containing
one allele of each gene fertilizes the egg containing one allele
of each gene, then the offspring will once again have two alleles
for each gene. As an example, let's say a breeder crosses a Black
dog (image 1) with a Gold bitch (image 3). For simplicity sake,
it is assumed that no recessive dilution alleles are carried by
either parent. So, the Black sire has to be A/- because
he is Black, and the Gold dam has to be Ay/- because
she is Gold. The result yielded 8 puppies: 4 Blacks, 2 Golds, and
2 Black and Tans.
Why are there more Blacks than anything else? Where did those
Black and Tans come from? To answer these questions, the breeder
reasons backwards. If the breeding yields any Black and Tan puppies
at all, then considering that these puppies' genotype MUST be at/at ,
we then know that each parent must also carry at . So now the breeder
knows exactly what the genotypes of the parents must have been.
The Black parent was A/at and the Gold
parent was Ay/at . To find the ratios
of what the breeder should expect in the litter, a list of all
combinations should be made.
The Black male can produce sperm containing the A allele,
but he will produce an equal number with the at allele. The Gold
female can produce eggs with the Ay allele
and an equal number with the at allele.
What follows is a list of all possible outcomes.
A from father, Ay from
mother offspring will be A/Ay and
appear Black
A from father, at from
mother offspring will be A/at and
appear Black
at from father, Ay from
mother offspring will be Ay/at and
appear Gold
at from father, at from
mother offspring will be at/at and
appear Black and Tan |
Notice that the ratio is 2 Blacks to 1 Gold to 1 Black and Tan,
which is the same as the 4:2:2 that was yielded in the litter.
It is important to say that the 2:1:1 ratio is EXPECTED in the
offspring, but not guaranteed. Statistical variation will determine
what actually appears. Although many combinations are possible,
most litters from these parents will yield colors near that ratio.
A breeding between different Black and Gold parents could produce
either all Black puppies, or half Black puppies and half Gold puppies.
It is left to the reader to work out the genotypes necessary to
produce these results.
As another example, our breeder crosses a Black and Tan male having
medium toned tan points, with a Gold female like the one pictured
in image 3. Remember that the tan points on this male appear as
a medium tan because of the rufous polygenes (in this case, a balanced
checker collection). Assuming the Gold female (also with a balanced
checker collection) is Ay/Ay , and knowing
that the male is at/at ; what will the
breeder get? The male can only produce sperm with at and the female
can only produce eggs with Ay , so all
of the offspring will be Ay/at , and appear
gold, right? Well, yes and no. All the offspring will be Ay/at alright,
but because the male had a pattern of rufous polygenes that determined
his tan points should be medium toned (about equal numbers of red
and white checkers, to return to our analogy) and the female the
same medium toned Gold; the offspring could be anything from rich
Red (having randomly received lots of red checkers from both parents)
to Light Gold (having randomly received lots of white checkers
from both parents).
There was some prejudice against Light Gold in some Tibetan Mastiff
circles, mainly in Europe where this color was not seen in or produced
from the original few dogs that were imported there. (These cases
where an original small group of founders do not include an accurate
sample of the genes in the whole population are well known sampling
errors and are referred to as founder effects in genetics.) This
example above shows (and many breedings have confirmed it) that
you will often get Light Gold puppies from parents of "acceptable" colors,
carrying "acceptable" genes. All of these offspring in the above
cross are genetically identical Ay/at with
respect to the main color determinants, but they can exhibit the
full range of gold colors. Only variation in the modifying genes
determining richness of the tan accounts for the differences in
color. Because this is a polygenic trait, as evidenced by the continuous
nature of the phenotypes (no clear cut breaks in the colors of
light gold to dark red gold) the lighter color golds can be produced
in any litter. This supports the inadvisability of having color
prejudice against any color found in Tibetan Mastiffs in Tibet
(now China) or the rest of the range. It simply further limits
an already limited gene pool while discriminating against otherwise
sound dogs.
One last interesting point, it is theoretically possible (but
unlikely in the extreme, you would probably need to produce over
100 puppies to see all of the statistically unlikely combinations)
to breed a Black dog with a Gold dog and get all the possible colors
discussed in this article. For this to happen, however, the genotypes
of the parents would have to be A/at, D/d,
B/b for the Black parent and Ay/at, D/d,
B/b for the Gold parent. This cross will give Blacks, Golds,
Black and Tans, Chocolate/Browns, Chocolate/Brown and Tans, Blue/Greys,
Blue/Greys and Tans, Gold dilutes, Double Dilutes, and Double Dilutes
and Tans (and recessive blacks if those recessive genes are also
present). If the reader wants to work out the ratios, it is suggested
he/she finds Punnet squares in an old genetics text and makes up
one with 8 squares on a side. Good luck.
TMINFO note: There has typically been controversy amongst
breeders and fanciers when it comes to the Tibetan Mastiff
breed as dogs do come in a variety of colors. Especially
for breeding/showing purposes, unless and until all colors
are recognized the TMINFO volunteers suggest that it is
always best to check with the BREED STANDARD of your country
to determine whether the color of your Tibetan Mastiff
of choice is faulted/acceptable or not. |
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