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Agouti signal peptide (ASIP) or A locus is responsible for many different colour patterns in dogs. Specific A locus allele products can interact with functional MC1R and thus interfere with production of black melanin. Dogs have four different ASIP gene alleles with following dominance hierarchy: Ay > aw > at > a, meaning that the most dominant allele present will be expressed.

Genes on A locus (agouti) are expressed only if a dog on K locus does not carry KB allele (combination KB/KB or KB/n) and/or on E locus does not have e/e combination. In dogs that are able to express A locus the basic colour can be additionally modified by B or D locus. Thus the dog with a genotype b/b on B locus has all the pigmented areas changed from black to chocolate/liver brown (dog that is fawn/sable (Ay) is a livernose, while the at/at dog has chocolate-and-tan instead of black-and-tan coat colourin addition to livernose).

Ay allele is the most dominant allele and determines the fawn or sable coat colour. The dog that has both Ay alleles (homozygote Ay/Ay) or one in a combination Ay/aw or Ay/at or Ay/a will be always fawn or sable, if A locus expression is enabled by the E and K loci.

Wild type aw allele is responsible for switching between eumelanin and pheomelanin synthesis, causing that individual hairs have eumelanin and pheomelanin patches from base to tip. This phenotype is called agouti (wolf grey or wolf sable). Banded hairs are usually present along the dorsal area of the torso. The dog will express agouti colour when having the combination of aw/aw, aw/at or aw/a alleles, if A locus expression is enabled by the E and K loci.

Allele at is responsible for black-and-tan or tricolor phenotype. Black-and-tan dogs are basic black with brown pheomelanin on ventral region, legs, cheeks and as dots above the eyebrows (eg. Doberman). Tricolor dogs in addition to black and brown have white colour due to the absence of pigment in certain areas of the body (eg. Rough Collie). The amount and distribution of pheomelanin may vary between individual dogs and between breeds. A dog with two at alleles or one in a at/a combination will be black-and-tan or tricolor (if A locus expression is enabled by the E and K loci).

Allele a is the least frequent allele of the A locus and is responsible for recessive black or bicolor phenotype. It occurs only in shepherd breeds and is the only cause of black German Shepherd and bicolor Shetland Sheepdog. Because a allele is the most recessive of A locus alleles the dog must have two a alleles in order to express recessive black. The offspring will always have one a allele. In the case where an animal does not have black coat colour the result of genetic test reveals whether the dog carries recessive black allele.

Testing for A locus resolves which two of the four possible alleles is present. As mentioned above, the expression depends on the status of E and K loci.

 

Genotype Description
Ay/Ay: the dog carries two copies of the dominant Ay allele for fawn/sable; the offspring will always inherit one Ay allele.
Ay/ aw: the dog carries one copy of Ay allele for fawn/sable and one copy of allele aw for agouti phenotype; since Ay dominates over aw fawn/sable can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit Ay allele and a 50% possibility that the offspring will inherit aw allele.
aw/aw: the dog carries two copies of the dominant aw allele for agouti coat colour; the offspring will always inherit one aw allele.
aw/at: the dog carries one copy of aw allele for agouti and one copy of at allele for black-and-tan phenotype; since aw dominates over at agouti colour can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit aw allele and a 50% possibility that the offspring will inherit at allele.
aw/a: the dog carries one copy of aw allele for agouti and one copy of a allele for recessive black; since aw dominates over a allele agouti colour can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit aw allele and a 50% possibility that the offspring will inherit a allele.
at/a: the dog carries one copy of at allele and one copy of a allele for recessive black; since at dominates over a, black-and-tan can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit at allele and a 50% possibility that the offspring will inherit a allele.
at/at: the dog carries two copies of the at allele for black-and-tan coat colour; the dog is homozygote for at and the offspring will always inherit at allele.
Ay/at: the dog carries one copy of Ay allele for fawn/sable and one copy of at allele for black-and-tan or tricolor; since Ay dominates over at fawn/sable can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit Ay allele and a 50% possibility that the offspring will inherit at allele.
Ay/a: the dog carries one copy of Ay allele for fawn/sable and one copy of a allele for recessive black; since Ay dominates over a fawn/sable can be expressed; the dog is heterozygote with a 50% possibility that the offspring will inherit Ay allele and a 50% possibility that the offspring will inherit a allele.
a/a: the dog carries two copies of the recessive black a allele, thus the dog can express black or bicolor coat colour (depending of the breed) the dog is homozygote for a allele and the offspring will always inherit a copy of a allele.

 

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