October 12, 2007
I have only recently started learning about genetics but have found it really interesting. This article is just a short overview and more for my benefit really, as I tend to learn things easier by taking notes when reading information, then re-writing them. I just hope it makes sense!
A fancy rat has 42 chromosomes, which are made up of 21 pairs. Each pair contains one chromosome from each parent. It is the genes contained on these chromosomes that control the characteristics of the rat. The position of each gene on a chromosome is called a locus (more than one locus is loci). Each locus contains two genes – one from the rat’s father, one from it’s mother. When the same gene occupies both positions on the same locus it produces a rat that is homozygous for that particular trait. A heterozygous trait is one that occurs when genes occupy opposite positions on the locus. In practice this means that with a gene that is dominant, such as Agouti, a rat can be either heterozygous or homozygous for Agouti (either A/a or A/A) and still look Agouti, whereas if a gene is recessive, such as non-Agouti (Black), the rat has to be homozygous for that trait for it to have a physical effect on the rat’s appearance. Some loci have many genes, such as the H-locus which controls markings, while others such as Chocolate have only two (that have been currently identified anyway!).
You also get genes called modifiers – these do not have positions of their own but latch on to compatible genes and change their effects.
There are hundreds of known fancy rat genes and new ones are being worked on and perfected all the time. Recognised loci are assigned a symbol (that is actually a letter) for identification purposes. A dominant gene is assigned a capital letter, while the recessive counterpart has a lowercase letter. Genes also control physical traits such as coat type (ie: Rex) or ear position (ie: Dumbo) as well as colour.
There are several websites with comprehensive listings of the known rat loci, so I won’t bother copying them all out here. I would say the most comprehensive one is here at Hawthorn varieties page and this is the one I personally use as reference for the symbols. The NFRS Varieties pages also has the genetic symbols listed, though some of them are slightly different to the Hawthorn ones.