By Holly R. Steel firstname.lastname@example.org
The genetics of coat color and pattern isn't something that
most people put much thought into until they come up with
something that they find unusual. It seems I've always had
Jack Russells with unusual coloring or markings, so maybe
that explains my persistent fascination with the subject.
The first terrier I bought had Dalmatian-like ticking; the
second had a huge blanket of color on his back and the third
started going gray at eight weeks.
Recently, a friend of mine told me she had a tan and white
stud dog that was producing black and white puppies.
"The books say that's impossible," she said. And
she's right, sort of. I found an explanation for her stud's
rare talent, and have explained it in an article designed to
accompany this one called "Where did those black and
white puppies come from?" But in talking to several
dozen people about this aberration, I discovered that very
few people have a working knowledge of basic coat color
genetics. Not that you should, necessarily; it can be a very
dull subject. But if you are interested in understanding why
seemingly strange things happen, you have to understand the
This article is intended to explain coat color and patterns
in dogs, with an emphasis on Jack Russells. I am not a
geneticist or a veterinarian and I don't profess to be an
expert on the subject. I am simply presenting information
taken from Malcom Willis' "Genetics of the Dog"
and Clarence Little's "The Inheritance of Coat Color in
Dogs." Also of note is an article written by Professor
Sue Ann Bowling that is found on her Web site, http://bowlingsite.mcf.com/DogPage.html.
Bowling, a Sheltie enthusiast, has a series of great
articles on canine genetics and her opinions on coat color
are worth looking at
Canine coat color remains a subject that needs more study,
so don't expect this article or any others to answer all
your questions. I still haven't figured out exactly why my
bitch went prematurely gray.
A few words about genes
Genes determine everything about your dog, from the color of
his eyes to thickness of his toenails. Willis calls the gene
"the unit of inheritance." Genes are always paired
and they exist at certain locations (loci), on chromosomes.
Most people are familiar with the concept of dominant and
recessive genes -- where when genes are paired; one may have
properties that outweigh the other. But many genes have
several characteristics, allowing them to express themselves
in many ways. These are alleles of a gene, and they allow
for much variation in coat color and pattern. Keep in mind
that while a gene can have many alleles, a given dog can
only carry two of those alleles and he inherits one from
If a dog carries the same allele in both loci, he is
described as being homozygous for that allele. If a dog
carries two different alleles in a given location, then he
is heterozygous for that gene. Since recessive alleles are
hidden in the presence of more dominant (hypostatic)
alleles, a dog must be homozygous for a recessive gene in
order for that gene to be expressed.
The easiest way to see how genes work is to use a Punnett
square. Since each dog carries two genes at a given
location, you can use a simple four-square box to calculate
the statistical probability of what genes the resulting
offspring will carry.
For example, we'll take a look at the simple color gene B.
The B series determines whether a dog will be black or
liver. The allele for black is dominant B and the allele for
liver is recessive b.
Liver is a disqualifying color in Jack Russells, but let's
say you have two terriers that are heterozygous for B,
meaning they carry both the desirable allele B and the
undesirable allele b. The genotype, or
genetic structure, in that locus of both terriers is written
as Bb. The terriers appear normal (are phenotypically
normal) because the dominance of B gives them the necessary
black pigment and overwrites the existence of the recessive
liver gene.What is the probability you will get liver puppies from
breeding these individuals? Take a look at the Punnett
square in Fig.
BB: 25% are homozygous for the desirable B allele.
This is ideal, because you don't want more liver carriers,
but there's no way to tell these puppies from their siblings
that carry the recessive b unless you do a test mating.
Bb: 50% are heterozygous for B, just like their
parents. These dogs are registerable because they have the
necessary black pigment, but will produce liver (bb) puppies
when bred to other carriers.
bb: 25% are homozygous for the undesirable b allele
and are liver colored. These dogs are unregisterable.
Keep in mind that the Punnett square can only give you the
probability of occurrence. It doesn't necessarily mean that
if you breed one litter from these dogs that you will get
these results. It's possible that the entire litter could be
liver, or you could have no liver puppies at all.
To add to the complexity, some alleles are affected by
modifiers, minor genes that can't really be quantified.
Modifiers play a small, but visible, role in the expression
of color genes. They tend to amplify or diminish the action
of the genes they modify. For example, in genes that control
the amount of white on a dog, modifiers work to increase or
decrease the amount of white. In some genes that dilute
color, such as cch, modifiers are credited for
controlling the level of dilution.
Coat color and pattern
Each gene involved in color has at least two alleles, and
these groups of alleles are called series. There are nine
well-documented gene series that affect coat color and
pattern. A couple more are mentioned by Willis and Little (R
for roan and P for pigment), but there isn't enough data on
them to discuss. Not all of the series listed below are
relevant to the Jack Russell, but I am touching on all of
them so that you have a general understanding of color
The A series. This is the series with which most
people are familiar. The alleles below are described in
Willis' book and are listed in order of dominance.
A dominant black
ay dominant yellow (golden sable -- tan in
ag agouti (wolf gray)
as bicolor with saddle markings (black
saddle markings with extensive tan on head and legs, like a
at bicolor with tan-points (black with tan
markings on face, legs and underbelly, like a Doberman)
In order for a Jack Russell to be black, he must carry at
least one A allele. Black is dominant to all other colors in
the A series. The black is true, from hair root to tip, with
absolutely no tan hairs mixed in.
The ay allele that produces tan is recessive to
black and dominant to all the other genes in the series. The
ay allele can produce various shades of tan --
from deep sable with some black hairs mixed in to clear,
rich tan. Some Jacks that are ay have sooty black
markings on their muzzles and sometimes even ringing their
tan spots or going down the spine (Fig.
2). These terriers are not tricolors -- you can tell
from the placement of the color, which on the face is
frequently the reverse of tan-point. Little (Page 35)
theorizes that these sooty tans are not homozygous for ay,
but rather carry the tan-point gene, or ayat.
The as and at genes are where we get
our black and tan Jack Russells (the addition of white that
makes these dogs tricolor is dictated by another gene
series). Little doesn't agree that a separate allele
produces the saddle markings; he believes that all instances
of the tan-point pattern are caused by at and
that modifiers cause the variation in the amount of tan.
Willis disagrees, and based on his view of Airedales, Wire
Fox Terriers and Beagles, likely would argue that as
and at are present in the Jack Russell.
Both as and at produce the same basic
pattern. The body of the dog is black, with tan found at the
extremities. But saddle-marked dogs have significantly more
tan, as shown in Fig.
3 and Fig.
4. Most of these dogs are born with more black, but
it gives way to tan as they mature. Some Jack Russells with
this gene have just the faintest black marks at their
temples or at the root of their ears, and it is possible for
some individuals to have no black hairs on the head at all.
With the at tan-point allele, which is recessive
to all others in the A series, the tan markings are much
more limited (Fig.
5). Variation also exists here, with some
individuals having the tiniest tan spots on the cheeks and
eyebrows, and others with tan eyebrows, large spots on the
cheeks, and the underside of the ears (Fig.
6 and Fig. 7).
So here's a breakdown of the various A series combinations
found in the Jack Russell.
AA - black
Aay - black carrying tan
Aas - black carrying saddle markings (tricolor)
Aat - black carrying tan-point (tricolor)
ayay - tan
ayas - tan carrying saddle markings
ayat - tan carrying tan-point
asas - saddle markings (tricolor)
asat - saddle markings (tricolor)
carrying tan-point (tricolor)
atat - tan-point (tricolor)
The B series. This series consists of just two
alleles, listed in order of dominance.
b liver (chocolate, brown)
The dominant B gene produces the black color in terriers
that are A or at and gives tan Jack Russells
their black noses. Jack Russells that are bb have liver (or
brown) noses and liver coloring where black should be,
whether the terrier is A or at (Fig.
8). Liver dogs also tend to have lighter eyes. Jack
Russells with liver coloring are not accepted into the
registry, and most JRTs are BB.
The C series. This series affects melanin (pigment)
production and has five alleles.
C color factor which allows melanin to be formed
cd white coat and black nose with dark eyes.
cb cornaz (gray) coat with blue eyes
c albinism with pink eyes and nose
The dominant C gene allows for full expression of color,
according to Willis. Nearly all Jack Russells are
homozygous, or carry both genes for, C. The chinchilla gene
dilutes color, like the d gene does. But it has its greatest
effect on tan, producing a cream color. It is possible that
"lemon" Jack Russells get their light color from
the cch gene.
The D series. Another of the simpler series, this one
consists of two alleles, listed in order of dominance.
D intense pigmentation
d dilution of pigment
Most Jack Russells have full pigmentation and are homozygous
for D. Terriers that are dd, will have slate gray coloring
where black should be and cream coloring where tan should
be. These colors are uniform, present at birth, and not a
result of fading. Dilute terriers also are not accepted into
The E series. This series affects the expression of
Em superextension with black mask
E extension without black mask
e restriction of black pigment (creates recessive
Em is the most dominant in the series, but isn't
found in Jack Russells. You see it in Boxers and Pugs that
are fawn with black faces. Most Jacks are EE, with full
expression of black pigment, but sometimes you'll see a
brindle pop up. Brindle is the expression of black bars on a
tan background. These terriers aren't accepted into the
registry. The recessive e gene, when homozygous, restricts
all black pigment (except in the nose) and produces color
that can range from red to yellow (Fig.
9). The markings contain no black hairs whatsoever,
regardless of whether the terrier is A, ay or at.
Although rare, there are ee Jack Russells and they are
accepted into our registry because their noses are black and
their color classification falls under the broad term
The G series. This series affects progressive
G allows graying
g no graying
This is the type of graying that occurs in Kerry Blue
Terriers and Old English Sheepdogs. Individuals are black at
birth and become gray as they mature. This is not the type
of graying that we have seen in Jack Russells because it
requires a dominant gene -- in other words, at least one
parent must be gray. My bitch that went prematurely gray had
normal parents and some of her puppies have turned gray like
she has, but not all. Jack Russells are gg.
The M series. This series allows the merle and
harlequin pattern seen in Great Danes and Australian
Shepherds, among other breeds. The dominant M is required
for the merle pattern and it is not seen in Jack Russells.
So we know our dogs are mm.
The S series. This series allows white markings and
its alleles are listed in decreasing order of dominance.
S Solid. No white spotting.
si Irish spotting (white on muzzle, forehead,
chest, belly, feet)
sp piebald spotting
sw extreme-white piebald spotting
Although Jack Russells are mostly white, the white isn't the
base color of the dog -- it is actually spotting. The
colored patches on the terrier show the true base color of
the dog. Jack Russells get their white from the two piebald
genes, sp and sw and the amount of
white is largely dictated by modifiers. A dog with plus
modifiers will have more color, while a dog with minus
modifiers will have more white. The modifiers make it very
difficult to tell whether a terrier is homozygous for one of
the piebald alleles or has both. According to Willis and
Little, it appears that some alleles in the S series may
have incomplete dominance over others, and this likely has
much to do with modifiers. Willis (Page 72) says
"Generally sp will act as dominant to sw
but much will depend upon the modifiers present as to the
exact appearance of the spsw dogs.
With many plus modifiers, they may seem to be more like spsp
and with minus modifiers will show much more white."
10 attempts to illustrate the expression of white
caused by sp and sw and modifiers.
While modifiers help determine how much white is expressed,
they do not appear to determine location of the pigmented
areas. Willis, referencing other researchers, says that dogs
appear to have standard "centers of pigmentation."
He says on Page 71, "Burns and Fraser (1966) argue that
the most important centers of pigmentation in the dog are on
the ears and head and around the eyes such that these will
be the last areas to become white. The next most important
areas are at the root of the tail followed by patches on the
ribs and finally by areas on the loin and lower part of the
The amount of white can make it difficult to determine what
the base color of the terrier is. For example, a dark
tricolor terrier with just a few spots on his ear could
appear to be black simply because the white areas of the
coat cover all of the tan areas on the dog (Fig.
The T series. This series allows ticking.
T allows ticking
t no ticking
Some Jack Russells have ticking. It is a dominant trait, and
individuals pick up the small spots in the white areas of
their coats as they mature (Fig.
12). The color of the ticking is determined by the
base color of the dog. Ticked Jack Russells are either TT or
Tt, non-ticked Jack Russells are tt. The most pronounced
example of ticking is found in the Dalmatian. Dals get their
white from swsw and are TT.
Now that you know what each series does and how it is
represented on paper, you can write out an approximation of
what your dog's coat color genotype is. A typical tricolor
Jack Russell with an average amount of color would probably
look like this:
Or, because it's understood that the same terrier is
homozygous for certain traits that rarely appear in the Jack
Russell, you could shorten it to the most important genes.
With the typical Jack Russell shown above, you only need to
show the A and the S series:
Once you determine what the genotype is for two given dogs,
you can calculate what they will produce using a Punnett
I hope this article proves useful to some of you who weren't
sure about color inheritance. The books that contain this
information are out of print and information on how color
genetics relates to the Jack Russell can be hard to come by.
From a breeding standpoint, coat color isn't significant
unless you're producing something that isn't accepted by the
registry. Color and markings should be the very last
consideration when you're trying to produce a healthy,
hard-working, good-moving terrier with a sound temperament.
If you have any questions or comments related to this
article, you may contact me via e-mail at email@example.com
R. Steel is a journalist and Jack Russell enthusiast
who lives in Georgia, U.S.A. This article first appeared on
Jack Russell Terrier Club Web site in May 2001. All