This article is from the Spring 1999 AFRMA Rat & Mouse Tales news-magazine.
By Eva Johansson, Sweden
The wild colour of the rat is called agouti. Most of you have probably seen this colour. The gene that gives the rat this colour is the agouti gene. One mutation of the agouti gene is the gene that gives the colour black. Many a person may believe that that gene is called black, but that is all wrong. If you look closely at an agouti rat you will see that there already is present black pigment, mainly at the tips of the hairs. But how can just one mutation switch an agouti coloured rat into a self black?
Let’s look at how the wild colour agouti gene works. Look at an agouti hair: black/blue-grey at the base, then a red/yellow band of colour, and last a black tip. Three different bands of colour on one individual hair. How does such a hair come about? Well, since the hairs are produced in the hair sacs in the skin, and the colour of each hair is produced at the same time, the individual hair must first get the colour black, then switch to red/yellow, and then switch back to black/blue-grey. What happens if the switching mechanism gets destroyed? Then the hair first will get black colour, but since the switch is broken there will be no switch to red/yellow and no switch back to black/blue-grey. The hair will become all black! Or rather black at the tip and fading to blue-grey at the base—just as if there was not enough paint to go all the way.
Anyway, what I just described is how the non-agouti gene works. It turns off the switching of colours that makes the three banded agouti hairs. The agouti gene is the gene that uses the switches to produce the banded hairs.
So the gene that makes a black rat is not a true colour gene— it is a non-agouti gene—no more agouti.
The genetic way to describe genes is a genetic alphabet. In this alphabet the agouti gene is called A and the non-agouti gene is called a. The a and the A both belong to the A-locus, the agouti locus. A locus is a special place among the heredic material for a special gene. This special place only holds versions of the A gene. Only two different A genes are present in the pet and fancy rat so far: A and a. In the mouse, several different genes in the A series are present, like the tan gene and the dominant yellow, besides A and a. The A series is just another name for all the genes that may belong at the A locus.