The A locus bicolor gene
This is part of a multi-page html document by Bonnie Dalzell, © 1997

A locus bicolor a-t a-t

The a-t gene on the a locus is characterized by the presence of black hairs on the dorsally promoted portions of the animal and red hairs with little or no eumelanine on the ventral portion, muzzle, cheeks, inside of ears and lower legs and feet. The demarcation between the dorsally promoted portions and the lighter ventral portions is generally quite sharp.

a-t is recessive to a-y, that is two dogs that are phenotypically a-y can produce an a-t dog. However the dominance of a-y appears to be incomplete in many cases because the a-y, a-t dogs frequently have a much heavier scattering of hairs on the doirsal regions of their bodies than do the a-y, a-y (clear red) dogs. a-t dogs are quite black at birth. Generally one can find pale fawn at the underside of the base of the tail and around the anus but the balance of the puppy may appear solid black. The reason for this is that almost all the hairs in a a-t dog are tipped with black. If one plucks hairs from the cheeks of an adult a-t dog and looks at them on a piece of white paper they generally have tiny black tips. So only the tips of the hairs are visible when the puppies are born. As they mature the hairs in the ventrally promoted regions have so little black at the tips that they give those areas their characteristic red or fawn color. The tops of the knuckles toes and small areas on the ventral midline as well as fur around the testicles and vagina may remain black on a-t dogs.

The degree to which the black color extends into the tan areas is also under genetic control and can be modified towards blacker or towards more tan depending upon the presence of other genes. Some of the more diluting genes of the chinchilla locus appear to influence a-t

A-t dogs may loose some of the black color with age and even grow fawn or red based hairs in the black portions of the coat.

a-t dogs have black noses, eye rims and lips unless there is some dilution gene present to pale out the eumelanine. a-t is variously referred to as black and tan, tan point, copper pointed. If the dog also has white markings it is often called tricolor. Some geneticists would like to call this color bicolor since this allows naming the color in dogs in which the eumelanine is diluted to chocolate or blue and also in dogs in which the eumelanine is fully present but the pheomelanine is dilute to pale cream or champaign. The confusion with this otherwise reasonable name for the color is that in many breeds dogs that are black with white markings but no tan are called 'bicolor'.

The dog on the left is a female Borzoi around 4 years old and out of coat. She has some paling genes that have diluted the pheomelaine in her tan points to a cream. She also has white markings on her face and feet (She is what dog fanciers would call a tricolor). The dog on the right is a grade bred (possibly cross bred) doberman with good intensifity of his tan.

black and tan borzoi, white trim black and tan doberman

The eumelanine in an a-t dog is susceptable to dulution. Below is a a-t who is also homozygous for the brown (chocolate) dilution gene b. b is on a different locus from a-t and is inherited independently. chocolate doberman
A sampling of Breeds in which a-t is common.
Collie breeds
Black and tans, tricolors = a-t with white trim
Black and tans, tricolors = a-t with white trim
black and tans
Doberman Pinscher
the entire breed
the entire breed
Bernese Mountain Dogs
the entire breed

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