Mastiff Coat Colour

Provided by Gwenstone Mastiffs with the kind permission of David F.Collinson, Ph.D.

Mastiffs have a range of colours more restricted than that found in many other breeds. Nevertheless, Mastiffs of unusual colouring sometimes turn up in the breed ring. On the one hand, there is a detailed listing of colours in the standard, but, on the other hand, there is no disqualification based on colour, so the judge may be in a quandary about whether (or how much) to penalize such a Mastiff. Similarly a breeder may be in doubt whether to use an otherwise outstanding animal with a distinct colour fault. To add to the problem, colour always has an element of interpretation; for example, some judges might consider a coat of Irish Setter-red'' a form of apricot, and others might not. Nevertheless, the colour of each individual has an unambiguous genetic code, if we are able to read it.

Very little has been published on the genetic background of the coat colours appearing in Mastiffs, so l thought a review of the current state of understanding of dog colour inheritance, applied to Mastiffs, might be in order to give the judge, the ringside observer, and the breeder or owner at home some insight about how what you don't see may affect what you get.

The bulk of the knowledge of the genetic basis of coat colour in dogs was provided by Clarence C. Little, Sc.D., who for 25 years worked in, and later owned, the 20 -100 dog private kennel founded by his father, and who for 16 years did experiments in breeding for coat colour at the Jackson Laboratory, with their stock of 200-220 dogs of 28 breeds, producing over 4100 puppies. He published Tbe Inheritance of Coat Colour in Dogs in 1957. I will be following Dr. Little, and supplementing it with the more recent information provided by Roy Robinson, F. 1. Biol. in Genetics for Dog Breeders (2nd ed.), 1990.

First I will give a brief summary of the mechanics of inheritance. I'm sorry to bore those who know this very well or who have no interest in it, but bear with me for a moment and I will try to avoid technical jargon as much as possible.

Within all the cells of nearly even living thing are strands, known as chromosomes, of a material called DNA, consisting of sequences of four chemicals. Each individual has just one set of sequences out of the almost limitless number of possibilities. The vast majority are required just to say "I am a cactus" or "I am a dung beetle'' or "I am a dog'' and the rest fill in the details of which cactus or dung beetle or dog the specimen actually is. The

chromosomes are arranged in pairs, and except for the fine details, the two members of a pair have the same structure. Along each of the pair are corresponding stretches called loci, and the specific arrangement of the chemicals occupying any locus is called a gene. The members of the set of genes that could possibly occupy a specific locus are called alleles of each other. In effect they are members of a gene family. Each locus of each chromosome relates to some structure or function of the creature, and, since they are paired, there is an another locus on the other member of the pair relating to the same thing.

From time to time I will indicate a pair of genes at corresponding loci by putting the symbols for the genes together, for instance q1 q1 would mean that each loci contained gene q1. What happens if the paired loci contain different alleles (perhaps q1 q2), each telling the creature to have different characteristics? Well, either there is a combined effect, or one allele is dominant over the other and determines that characteristic on its own. The other Allele, said to be recessive, has no visible effect on the animal or plant in question, but could have an effect on its offspring.

The reason for the pairing becomes clear when sex cells are produced, because they get only one member of each chromosome pair, so when they combine with a sex cell from another specimen, the resulting offspring has a new combination of gene pairs (say q2q3). The result is that at any given locus, an allele which formerly was the recessive member of a pair may now have a partner it is dominant over. Therefore characteristics may appear in the offspring that were not apparent in either parent.

Let's look at what is known about the coat colour genes in Mastiffs.

The Main Genes

Arguably the most important coat colour locus in dogs is the A locus. In decreasing order of dominance, the alleles at this locus are A^S, A^Y, A, a^sa, a^t and a. The last one, recessive black, is known only in German Shepherds, and we can forget about it. The only gene among these alleles that conforms to the Mastiff standard is A^y,.which, depending on other factors, can produce a dog anywhere in the colour range from pale biscuit through yellow to deep red. An important point about this gene is that it does permit the existence of some black hairs, usually identified as sailing, especially on the back, and it also permits a black mask and ears.

What about the other alleles?

A^s tries to produce an all black coat, providing the genes at other loci permit it. True blacks have historically occurred in Mastiffs and occur still, but they are a non-standard colour.

The gene called A produces the wild, or grey, colour, with hairs that are individually banded with black and another colour. This is probably the original colour of the dog species.

a^sa is a variation of this where these hairs are restricted to a saddle-like pattern on the dog's back and sides. Both these variations have been seen in Mastiffs, though they may be due to some other genes l will mention later. Recently I have seen a photograph of a Mastiff apparently carrying the a^t gene, which causes the black and tan pattern, as is seen in Dobermans and Rottweilers. None of these genes give a coat colour approved by the modern standard.

The next most important locus for coat colour is the E locus. This is not quite so well understood as the A locus, although it has been much studied. Little believe the dominant gene was E^m, a gene which attempts

to produce a black mask and ears, but restrict the rest of the coat to the yellow-red range. These days, most people believe E^m does not exist, but that the mask comes from a dominant gene, Ma, at a separate locus. The no-mask recessive would be ma. Whichever gene causes the mask, most Mastiffs have it. If E^m does exist, it has to share dominance at this locus with E^br the brindle gene. Little said that when a dog has the gene pair E^mE^br, It shows both a black mask and brindling. Two such dogs bred together could have some offspring of type E^brE^br which presumably would not have a black mask at all. This is the problem with E^m, because no such breeding of masked brindle dogs producing maskless brindle dogs has been reported. Really the only hope for the continued belief in E^m is the possibility that the pairing EbrEbr itself produces a mask because of all the dark pigment it generates.

Next lowest in dominance at this locus is E, the gene for extending black pigment over the whole body. Surprisingly, if E^m does not exist, then this is the gene possessed by non-brindle Mastiffs. The reason it does not normally produce black Mastiffs is that Mastiffs, as we have seen, carry the A^y gene, which does not provide much black pigment for E to spread, except for possible sabling.

The final gene at this locus is e, the non-extension gene. Because it is recessive to the others, the gene pair has to be ''ee'' for it to have any effect, but in that case, it produces clear yellows or reds with no black hairs at all. There is no indication that this gene exists in Mastiffs, and if it did, it would probably produce specimens with no mask.

One gene locus important in Mastiffs but obscure for most other breeds is the C locus. This is the locus for so-called chinchilla silvering, represented by gene c^ch, which gives a light, flat tone to the non-black pigments, but leaves black unaffected.

Hence we have the silver-fawn colour. The dominant gene here is C, the gene for full colour, and the only one known for most breeds. Many Mastiffs show no silvering, so this gene is not uncommon in the breed.

Another possible gene is c^e, representing an extreme silvering, approaching white. lf this gene exists, it might be responsible for the lightest coloured Mastiffs. There are two other genes at this locus, c^b and c, blue-eyed albino and true pink-eyed albino, but these are very rare in any dog breed and not known in Mastiffs.

Rare Genes

A locus which is important in some breeds but not usually in Mastiffs is the B locus. Mastiffs generally have only the dominant B gene, which allows the black colour in masks, ears, brindle stripes Etc. The recessive b gene, which turns those black areas into brown, does crop up from time to time, most famously in Crown Prince, a dog from 100 years ago who is at the back of every extended pedigree.

The D locus is another which rarely figures in Mastiffs. The full-colour gene is called D. The recessive gene d dilutes black pigment into a bluish grey, and also lightens and dulls reds and yellows. Again it has been known

to crop up from time to time in Mastiffs, and is particularly noticeable in the so-called blue brindles. It is possible for this gene to co-exist with the previously mentioned b gene, and dogs with gene configuration bbdd are technically known as lilac. This is usually only seen in the Weimeraner, the isabella Doberman and possibly some Chesapeake Bay Retrievers, but I have seen a Newfoundland in a washed-out brown shade which might have been a lilac.

The S locus (affecting spotting) is one which might have been quite important in the past, when large dogs which fell under the general description of Mastiffs were often pied or even predominantly white. It is difficult to say how much such dogs contributed to the breed we have today, but they have certainly been strongly selected against since the last century, perhaps to avoid confusion with St. Bernard's.

S is the normal, no-spotting gene, then, in descending dominance, come: sⁱ, causing white underneath, legs and chests, with a white collar and blaze; s^p, causing piebald spotting, or patches on a white background; and s^w, causing a few small spots on a white background. All these have certainly greatly declined in Mastiffs, and now white areas come overwhelmingly from genes I will deal with in the next section.

Polygenes

The important characteristics of living creatures usually are controlled by a single locus. This makes the process of ''natural selection'' far more efficient and allows these characteristics to evolve more rapidly, and thus help the species survive changes in the environment. Alternatively, some less essential characteristics may actually be controlled by the alleles at a number of loci, called polygenes. This is the case for an undetermined number of colour characteristics. These polygenes take the form of plus or minus modifiers, which vary the expression of a characteristic whose basic form has been set by some major gene.

The most researched set of colour polygenes in dogs are those that vary the expression of the genes found at the S locus, which controls spotting. Ignoring the very rare albino and blue-eyed albino genes, there are four main genes at this locus, but it is easy to distinguish at least ten degrees of spotting! The "fine tuning" is done by a number of genes at other loci, each of which can be either a "minus modifier'' which tends to reduce spotting, or a ''plus modifier'' which tends to increase it. In Mastiffs, the genes other than 5, the solid colour gene, have been eliminated, or nearly so, but plus modifiers, if there are enough of them, can cause white areas on feet, chest, and sometimes elsewhere, even approaching the extent of the next lowest degree of spotting. Like other polygenes, for instance the ones that effect hip dysplasia, it will be extremely hard to eliminate these genes until a genetic test is found, because when you mate two animals with mostly minus modifiers and a few plus, chance can lead to all the plus modifiers ending up with one or more of the offspring, and you get some pups more affected than either parent. The simple rules of breeding are thus much less effective when you are dealing with polygenes rather than genes which all appear at one locus!

The rufus polygenes

The most significant, well-established set of polygenes found in Mastiffs, other than the spotting polygenes which have been dealt with above, are the rufus polygenes. These determine whether the coat is fawn or apricot. The true nature of the genetic basis of the fawn-apricot divide has certainly led me into fruitless arguments in the past, and I am grateful to Robinson (previously cited) for this clarification. As well as any single major coat-lightening genes, such as the silvering gene c^ch mentioned previously, there are a set of polygenes in which the minus/plus modifiers tend to make the coat paler, e.g. fawn in Mastiffs, or deeper, e.g. apricot in Mastiffs.

Research indicates that on an individual gene basis, the fawn may be dominant over the apricot, but a specimen with plus modifiers at a majority of loci will be apricot, the more plus modifiers, the deeper. It may be that only those individuals that lack the c^ch gene can achieve the true red so much more common in Bullmastiffs than Mastiffs.

The umbrous polygenes

It has already been mentioned that the type- of light pigment present in Mastiffs allows the presence of some dark tipped hairs, or sabling. The extent of this can amount to visually nil or very heavy shading, and this variation is attributed to the umbrous set of polygenes. These genes can lead to the so-called "smutty" coat. These polygenes may also have an effect on patterns such as brindling or the saddle pattern, but these probably have there own sets of polygenes. The saddle pattern seen on some dogs, including a few Mastiffs, may be the result of polygenes alone. In Mastiffs there may be polygenes affecting the width, frequency, intensity, and evenness of brindle stripes, but no research has been done. Other polygenes may affect mask. There are probably lots more sets of polygenes, but they are very hard to research, as their inheritance modes are so much more complex than the major genes.

Genes Not Found ln Mastiffs

There are some gene loci where Mastiffs are only known to have a standard "normal, solid colour'' gene. Nevertheless, through mutation or from long ago cross-breedings, some other genes could crop up in the future. There are several types of mixed colour coats other than the ones already mentioned, including merle, a mingled or patchy pattern of light and dark areas, and two related patterns: harlequin, where an extra gene makes the light areas into white, and tweed, where a different extra gene makes the light areas have different intensities. There is also ticking, where there are spots of mixed coloured and white occurring in otherwise white areas of the coat. (This might be one to watch out for, since it is common in Saint Bernard's, the main source of untypical genes in Mastiffs.)

lf this pattern occurs throughout the coat it is called roan, and if the spots contain no white hairs, it is called flecking. As well as the D locus dilution of colour, which is not unknown in Mastiffs, There are several other colour-diluting genes, including pink-eyed dilution, progressive silvering dilution, in which black areas become grey with maturity, and powder-puff dilution, only reported in Collies so far, in which grey areas become black. Also only in collies are a dominant slate-grey dilution, and CN dilution, where grey colour is just a feature of a fatal genetic disease.

The genes of the typical Mastiff

Finally, we can now describe the genetic basis for the typical Mastiff coat. I will use an abbreviated notation. All the loci for unusual colour genes will just have a ''normal, solid colour'' gene, so l make no further mention of them. Remember that every gene is a partner in a pair of loci with an identical range of possibilities. When l write a single gene. l will be representing an identical pair, and when l write a choice in brackets, e.g. (E or E^br), then l mean the pair of genes may both be E, or both E^br, or they may be one of each. Commas indicate a change of locus pair.

The standard Mastiff colour gene combination is:

A^Y, B, (c^ch or C), D, (E or-E^br), Ma, S

This symbolises a dog in the yellow/red colour range, with any dark pigment being black, possible ''chinchilla'' silvering, no dilution, either solid coloured or brindled, a black mask, and no major white areas.

We also know that there are polygenes which determine whether the dog is at the yellow end (fawn) or the red end (apricot) of the colour range, while other polygenes determine how much dark pigment (sabling) there is, how extensive any minor white spotting there is, and what the character of the brindling is, if present, as well as the extent of the mask.

To go from the normal to the most extreme, from known variations in the Mastiff colour range, we have seen that dogs can exist which are entirely black, or lacking any black markings, or substantially white. Given an

unlikely combination of rare genes, a black and white patched Mastiff is possible. Furthermore, the black in any colour pattern could be replaced by brown "blue'' or ''lilac''.

For instance, A^s,b,c^ch,d,E,ma,s^p would be a markless white dog with pale ''lilac'' patches ! l emphasize it would be virtually impossible to gather together all the aberrant genes needed to produce such an animal.

In conclusion, Mastiffs are a breed with a limited variation in colour, nothing like as complicated as Poodles or even Great Danes. No one is likely to see the most extreme possible combinations of unusual genes, unless some unscrupulous person resorted to cross-breeding to more variable breeds like Neapolitan Mastiffs. It therefore remains with the breeder and the judge to take into consideration the overall animal when considering how much ''weight'' to apply to individual colour faults.

By David F.Collinson, Ph.D.

HOME