Genetics of the dog’s coat

Genetics is not always an easy subject and, while some breeders who have worked in genetics a long time understand it and are perfectly able to interpret what they see and even to provide for scientifically grounded mating plans, others feel very unsure about the whole matter. A good example of this approach is the study of the genetics of dog coats.

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Pigment and loci of action

A class of pigments known as melanin is almost exclusively responsible for hair, skin and eye (iris) colour in mammals. Melanin exists in two forms: eumelanin is a black-brown pigment and phaeomelanin is a red-yellow pigment. The hair cortex is translucent, so in the absence of these pigments, the air inside it gives it a white appearance – the same phenomenon occurs in snow, where air is trapped between the ice crystals.

A certain number of alleles at different loci may participate in:
• Synthesis of eumelanin in all or part of the skin and coat
• Divergence of this synthesis towards the synthesis of pheomelanin in all or part of the skin and coat
• Inhibition of melanin formation in all or part of the skin
The loci in question can be categorised as follows:
• Loci that determine the base colour of the coat
• Loci affecting the intensity of the pigmentation
• Loci for piebald

© Diffomédia/Royal Canin

Locus determining the base colour

The base colour of the dog’s coat is determined by the action of the gene at three different loci: B (“black”), A (“agouti”) and E (“extension”). The variety of these alleles is the cause of the heterogeneity of colours in dogs.

B locus (“Black”

There is no debate here. The two alleles are:
B+: eumelanin is black
b: eumelanin is brown
The shade of brown can vary under the influence of modifier genes or due to interactions with other colour genes but it will always be recessive compared to black.

A locus (“Agouti”

Illustration_Zoom_Poils_Agoutis

The various mutated alleles at the A locus modify the extension of eumelanin and pheomelanin in each hair and in the coat as a whole.

The number and nature of the alleles at the A locus are not the same, but it is possible to present a highly probable A locus, proposing useable alleles without any great risk to interpret most situations.

Illustration_As_A+_Ay

As (s for “self”). is qualified by “dominant black”. In the overwhelming majority of cases this is responsible for uniform black.

A+, commonly known as “grey wolf”, governs these types of coat in some Nordic breeds (Elkhound, Keeshond). A+ is said to correspond to the coat of the wild form of the dog, the wolf.

ay (y for “yellow”) is responsible for dark fawn. This widespread colour, close to the wild coat, has become highly polymorphous based on the amount of black marking, from very slightly darkened fawn to very dark fawn depending on the quantity of polygenes accumulated by selective breeding.

Illustration_asa_at

asa (sa for “saddle”) governs fawn saddle which may produce anything from a small saddle to a black coat with fawn markings.

at produces “black with fawn markings”

E locus (extension)

The E locus controls the relative distribution of eumelanin and phaeomelanin, but it appears to work on the coat as a whole rather than individual hairs. The wild E+ allele does not express itself, allowing the A alleles to express themselves instead. There are three other alleles at this locus: Em, ebr and e.

Illustration_fauve_masqué

Em tends to concentrate eumelanin on the face, although it is scattered on topline, tail and even chest. The overall effect is “masked fawn”

Illustration_ebr

ebr modifies the distribution of black markings by arranging them in brindle form.

e totally eradicates eumelanin in the coat, producing a uniform fawn colour. E suppresses all A alleles.

Locus affecting the intensity of pigmentation

C locus (“Colour”

The colour series is sometimes also known as the “albino” series. There are at least three alleles at this locus: C+, cch and c.

C+, the “colour gene” is not expressed, allowing genes at other loci to act.

c, as in other species, is said to produce total albinism (with “red” eyes). It is exceptional in dogs, although it has been described in Pekingese. The existence of a pseudo-albinism cb gene has also been advanced, in which animals are extremely pale, almost white, with depigmented extremities and “blue” eyes. This rare phenomenon has been observed in Dobermanns. This cb allele, which is dominant over c, may well exist.

Illustration_cch

cch dilutes fawn pigment, turning it a sandy colour. The homozygous cch gene produces the extreme dilution of clear pigment.

D locus (“Dilution”

Illustration_cch_dd

The D series also acts on the intensity of pigmentation, although the mechanism is different. The number of pigment granules is not reduced, rather they are typically agglomerated in parcels which reduces the absorption of light, producing a paler colour, such as blue instead of black.

Two alleles have been identified at this locus. D+, is not expressed, allowing genes on other loci to act, whereas recessive d dilutes eumelanin and phaeomelanin, so that black turns to blue, and brown, beige and fawn turn sandy.

G locus (“Greying”

Illustration_Grisonnement

It is generally accepted that there are two alleles at this locus. In animals born with a non-diluted coat colour G provokes the gradual appearance of whitish hairs, which mix closely with the coloured hairs to lighten the coat to a greater or lesser degree.

The wild G+ allele has no effect. The G gene is fairly widespread in the canine species. The essential criterion for its intervention is the gradual modification of a coat that is normally coloured at birth. If it combines its action with other dilution genes (such as c or d), it can produce a wide variety of shades. However the idea of several greying genes is gaining ground

M locus (“Merle”

Illustration_Merle

According to the traditional hypotheses, there are two alleles at this locus. Mis responsible for coloured patterns, whereas M+ has no effect. Heterozygous M has a very clear action on a dark coat (eumelanin), lightening or mixing the base colour while leaving the basic pigment, more or less speckled, producing:
• Blue (or grey) merle in interaction with As
• Traditional blue merle with fawn markings in interaction with at
• Beige merle and beige merle with fawn markings, which are rare (found in the Australian Shepherd).

M has a much more discreet effect on pheomelanin. Here, the contrast between the merle and the base coat is only apparent in puppies and is not usually detected in adult dogs. As a result, fawn adults may pass on the merle gene.

Homozygous M can lead to total depigmentation (in some Great Danes) or the appearance of sometimes-intrusive white patches on dogs that do not carry a piebald gene (Dachshund).

Piebald locus

S locus (“Self”

Illustration_Panachures

Most authors accept the existence of four alleles at this locus, in the following order of dominance:
S+: uniformly coloured coat
si: Irish piebald (corresponding to limited
sp: irregular piebald
sw: intrusive piebald

These alleles maintain a relationship of incomplete dominance, although not all the time, and are also strongly affected by the action of modifier genes, causing the limits of one class to be confused with those of the neighbouring class.

T locus (“Ticking”

Krasowski_Fotolia

Many piebald individuals present with more or less abundant ticking in white areas of the coat. This is due to a dominant allele at the T locus. The abundance of ticking is said to be polygenetic.
The T gene does not express itself at birth, so ticking and mixing only appear gradually.

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