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 Ay mice through the banded coloured hairs of A to no yellow at all coat of the ae 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 401 (2/2014) reported genes (alleles) in the a locus, of which 296 are products of gene trapping. 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. Ay - Lethal dominant yellow
  2. Avy and Ahvy - Viable yellow and hypervariable yellow
  3. Aw - White-bellied Agouti
  4. A - Agouti or Wild type
  5. at - Black and Tan
  6. am - mottled agouti
  7. a - Non-agouti
  8. ae - 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/at x a/a cross.
  • An agouti mouse was produced from a Ay/a x a/a cross.
  • A black tan mouse was produced from a A/* (non A/at) x a/a cross.
  • There has been numerous reports of at and Aw mice appearing from a/a x a/a crosses, most likely due to remutations of a.

Aw and at are the most common "targets" of remutations.

A large number of alleles in a-locus are, in fact, remutations to already existing alleles. There are 21 different at's listed, of which 20 are spontaneous mutations and 20 different Aw -mutations.