Glossary

By Holly R. Steel hsteel@mindspring.com

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 basics.

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 
(http://bowlingsite.mcf.com/Genetics/ColorGen.html).

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 each parent.

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.

Illustrations

Fig. 1
Punnett square

Fig. 2
Sooty markings

Fig. 3
Saddle markings

Fig. 4
Saddle markings

Fig. 5
Tan-point

Fig. 6,7
Tan-point

Fig. 8
Liver

Fig. 9
Recessive red

Fig. 10
White spotting

Fig. 11
White spotting

Fig. 12
Ticking

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. 1.

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 c^ch, 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 genetics.

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
a^y dominant yellow (golden sable -- tan in Jack Russells)
a^g agouti (wolf gray)
a^s bicolor with saddle markings (black saddle markings with extensive tan on head and legs, like a German Shepherd)
a^t 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 a^y allele that produces tan is recessive to black and dominant to all the other genes in the series. The a^y allele can produce various shades of tan -- from deep sable with some black hairs mixed in to clear, rich tan. Some Jacks that are a^y 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 a^y, but rather carry the tan-point gene, or a^ya^t.

The a^s and a^t 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 a^t 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 a^s and a^t are present in the Jack Russell.

Both a^s and a^t 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 a^t 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
Aa^y - black carrying tan
Aa^s - black carrying saddle markings (tricolor)
Aa^t - black carrying tan-point (tricolor)

a^ya^y - tan
a^ya^s - tan carrying saddle markings (tricolor)
a^ya^t - tan carrying tan-point (tricolor)

a^sa^s - saddle markings (tricolor)
a^sa^t - saddle markings (tricolor) carrying tan-point (tricolor)

a^ta^t - tan-point (tricolor)

The B series. This series consists of just two alleles, listed in order of dominance.
B black
b liver (chocolate, brown)

The dominant B gene produces the black color in terriers that are A or a^t 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 a^t (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
c^ch chinchilla.
c^d white coat and black nose with dark eyes.
c^b 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 c^ch 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 our registry.

The E series. This series affects the expression of black pigment.
E^m superextension with black mask
E extension without black mask
e^br brindle
e restriction of black pigment (creates recessive red/yellow)

E^m 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, a^y or a^t. 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 "tan."

The G series. This series affects progressive graying.
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)
s^p piebald spotting
s^w 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, s^p and s^w 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 s^p will act as dominant to s^w but much will depend upon the modifiers present as to the exact appearance of the s^ps^w dogs. With many plus modifiers, they may seem to be more like s^ps^p and with minus modifiers will show much more white."

Fig. 10 attempts to illustrate the expression of white caused by s^p and s^w 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 back."

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. 11).

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 s^ws^w 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:
a^ta^tBBCCDDEEggmms^ps^wtt

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:
a^ta^ts^ps^w

Once you determine what the genotype is for two given dogs, you can calculate what they will produce using a Punnett square.

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 hsteel@mindspring.com

Holly R. Steel is a journalist and Jack Russell enthusiast who lives in Georgia, U.S.A. This article first appeared on the
Georgia Jack Russell Terrier Club Web site in May 2001. All rights reserved.