Rat color genetics are extremely simple. It’s really disheartening how many of the breeders out there simply do not know what produces what, or are unable to identify the colors on their own litters.
Most rat colors are the result of a double recessive gene on a single locus. The more loci involved, the more complicated it becomes, and the harder identification of a rat by sight alone becomes. That is why it is so important to keep good, complete and consistent pedigree information, and to try to work with as few extraneous dilutes as possible.
The basic information for working with rat genetics:
Gene – the genetic information passed from parents to child, one from the mother and one from the father, which affect rat color
Locus – plural loci, this is the paired gene location. Each locus affects a different type of dilution.
Dominant – indicated by a capital letter in a gene pair (A/a for example) this gene is dominant and will prevent the recessive from expressing. There are very few dominant genes which have expression in rats – most of the time, you are looking for the recessives.
Recessive – indicated by the lower case letter in a gene pair (A/a for example) this gene will only express if both genes are recessive. However, a rat can be a “carrier” for the recessive gene, as in the example, and has the possibility of throwing double-recessive babies when bred to another rat with the same recessive.
Homozygous – A matching pair of genes, with both being either dominant or recessive. An A/A rat is homozygous for the Agouti gene.
Heterozygous – A pair of genes with one dominant and one recessive or carried, gene. An A/a rat is heterozygous for the Agouti gene.
Punnett Square – used to work out the chance of a single locus to express in a given mating. The father’s genes are listed on the horizontal of the table, and the mother’s genes along the horizontal. Example:
In the following example, both parents are heterozygous Agoutis. They have a 25% chance of producing homozygous Agoutis, a 50% chance of producing heterozygous Agoutis, and a 25% chance of producing Black-based babies.
Genotype – the genetic color identification of the rat. There are rats who can appear very similar to another color, despite not sharing color genes with them.
Phenotype – the color the rat appears to be. This can be quite different from the genetic identity!
Expressed – when the color gene is visibly affecting the color of the rat
Carried – when a recessive is hidden by the expressed color genes, but is still present and may express in offspring when bred to another rat with those genes.
Modifier – A little “tag” on the locus which tells the gene to produce a slightly different shade, while genetically remaining the same color as another rat without the modifier. This can cause endless confusion when attempting to identify babies, and is where selection to standard comes in.