The eye colour of a rat will also drastically change the coat colour too. There are controlled by two different recessive genes, pink-eyed, P, and ruby-eyed, R.

It is worth knowing that other rats in the agouti and non-agouti groups will undergo a similar change in coat colour. For example, a cinnamon will look like a silver fawn, albeit a slightly lighter version, when it has pink eyes.

This may help illustrate the point that all because a rat has the key genes to make it a certain colour, other genes and variations may mean the colour isn't a show winning one. This is one reason breeding is more than just simple genetics.

There are number of other effects that can be explained by more involved genetics. I can't hope to cover them all fully, but I'll say enough to make the interested aware that they exist.

We had a black rat carrying ruby eyes. The coat colour wasn't really black, and was like a half-hearted chocolate. The reason for this is that the ruby-eyed wasn't completely dominated by the black-eyed gene, and this caused the slight colouring.

Compare this with complete dominance covered earlier on page 2.

A more common example of incomplete dominance is the rex gene, RE. All rexes at shows are REre. The rere would be what is called double-rex or an extreme rex. This combination causes the rat to be have extremely curly fur. Double-rexes tend to be quite bald and feel the cold, so crosses that would result in rere kits are often avoided.

The genes looked at so far have either been one of two possible sets of information. Examples would be information to be top-eared or dumbo-eared, roaned or not roaned, agouti or non-agouti, have curly hair or straight hair, and so on.

To make things more 'fun', the c and ch alleles are incompletely dominant to each other. So that gives us six geneotypes instead of the usual three.

The hooded gene, H, is similar in that there are a number of different alleles which control the patterning of marked rats.

The pearl gene, Pe, is a little unusual in that the normal, non-pearl characteristic is recessive and it is the pearl that is dominant. However, the pearl effect will only take effect when the recessive mink characteristic is present too. That is, pearl would be aa mm PEpe.

The pearl gene however is a lethal gene. Rats with homozygous form, PePe, either die shortly after birth or die before birth and are reabsorbed, so there tends to be less rats in a pearl litter.