The Agouti gene (ASIP gene) is responsible for the production of a protein that regulates the distribution of black pigment (eumelanin) within the hair shaft. This gene is also known as the A-locus and determines whether an animal expresses an agouti appearance, and if so what type, by controlling the distribution of pigment in individual hairs. The agouti pattern can be seen in both black-based and red-based colours. The coat colour is further complicated by the interaction with the K-locus and the E-locus. The agouti pattern is only expressed if on the K-locus no copy of the KB allele is present in combination with at least one copy of the E or Em allele on the E-locus. The Coat Colour A-Locus test (H820) tests for the genetic status of the A-locus. The A-locus has four variants (alleles). The most dominant allele is Ay, followed by aw, then at, then a. The dominant Ay allele produces a sable or fawn coat colour. The allele aw produces a colour known as wild sable or wild type. With this colouration, the hairs switch pigmentation from black to reddish or fawn. This colour is sometimes seen in German Shepherds and other shepherd breeds. The allele at results in tan points (tan markings on a dark dog) and produces black-and-tan and tricolour dogs. A tricolour dog is black-and-tan plus white. The allele a is also called the recessive black allele and results in a solid black/brown/blue/lilac or bicolour dog. Some breeds are fixed for only one variant. The Norwegian Elkhound is fixed for the aw allele and the Beagle is fixed for the at allele. In many breeds 2 or 3 alleles are present.
The Coat Colour A-Locus test encloses the following results.
| A-Locus | Coat Colour |
|---|---|
| Ay/Ay | Fawn/Sable, only allele Ay will be passed on to an offspring |
| Ay/aw | Fawn/Sable, either allele Ay or aw will be passed on to anan offspring |
| Ay/at | Fawn/Sable, either allele Ay or at will be passed on to an offspring |
| Ay/a | Fawn/Sable, either allele Ay or a will be passed on to an offspring |
| aw/aw | Wild sable/Wild type, it can only pass on allele aw will be passed on to an offspring |
| aw/at | Wild sable/Wild type, either allele aw or at will be passed on to an offspring |
| aw/a | Wild sable/Wild type, either allele aw or a will be passed on to an offspring |
| at/at | Tan Points/Black-and-tan/Tricolour, it can only pass on allele at will be passed on to an offspring |
| at/a | Tan Points/Black-and-tan/Tricolour, either allele at or a will be passed on to an offspring |
| a/a | Solid Black(Brown/Blue/Lilac)/Bicolour, it can only pass on allele a will be passed on to an offspring |
