A-series, or Agouti locus.

This series of alleles control the nature of the pigmentation of each individual hair of the mouse's coat, as well as the distribution of the pigmentation over the mouse's body. The A-series, or the a-locus, determines the relative amount and distribution of yellow pigment (phaeomelanin) and black pigment (eumelanin) in the hairs of the coat. This distribution goes from all-yellow (in the genetical sense!) coat of A^y mice through the banded coloured hairs of A to no yellow at all coat of the a^e mouse. While most other colour controlling other genes control the depth of this red or black pigment, this locus controls which pigment the hairs in the mouse’s coat has and in what amount. This makes the a-locus the most important one.

There are currently 86 reported genes in the a locus. The locus is situated in chromosome 2. It is officially called "nonagouti", after the basic mutation from wild type: a.

Genes relevant to mouse fancy in this locus are:

1. A^y - Lethal dominant yellow
2. A^vy & A^hvy - Viable yellow & hypervariable yellow
3. A^w - White-bellied Agouti
4. A - Agouti or Wild type
5. a^t - Black and Tan
6. a^m - mottled agouti
7. a - Non-agouti
8. a^e - Extreme non-agouti

An interesting feature of the a-locus is the occasional occurence of nonparental phenotype young. Reported cases include:

• A homozyous a/a mouse was produced from an A/a^t x a/a cross.
• An agouti mouse was produced from a A^y/a x a/a cross.
• A black tan mouse was produced from a A/* (non A/a^t) x a/a cross.
• There has been numerous reports of a^t and A^W mice appearing from a/a x a/a crosses, most likely due to remutations of a.

In fact, a reverting to two more dominant alleles a^t and A^W is said to happen unusally often.