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Breeding mice for pets or show is a tricky business. It takes a lot more than simply putting two attractive mice together and hoping for the best, and even the most perfect mice can produce offspring you simply didn’t expect. A lot of research has gone into the genes that produce the various colours, markings, and coat textures that we desire in the fancy.
This essay aims to help people who - like me - are just starting out in understanding the genetics behind the beautiful mice we adore. As my knowledge grows, so this essay will continue to be revised and updated; and I hope it helps others who are looking for a simple and straight-forward guide to the genetics behind mouse breeding.
Note: words in blue appear in the glossary.
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1.1 Mendelian Inheritance
If you imagine DNA as a sort of zipper, each side “zips” together to form the twisted ladder we are all familiar with. One side of this ladder comes from the male parent, the other from the female parent.
At the simplest level, let’s pretend that each “rung” on this ladder represents a gene. Mendel theorised that if we know what the genes the parents carried, we could reasonably predict what genes their offspring would carry.
While inheritance is a lot more complicated that this in reality, Mendel’s model has been demonstrated to be consistently reliable in making predictions. These predictions are useful in selecting mice to breed when looking for certain characteristics.
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1.2 Mouse Genes
There are a lot of genes that affect a mouse’s physical appearance. This essay only covers the most prominent of these genes, the ones that breeders see most frequently. If you are interested in the vast number of different genes and mutations seen in fancy mice, I recommend the book The Coat Colors of Mice, the details of which is in the references section at the end.
Each gene represents a single aspect of the mouse’s appearance, and is formed by the pairing of a locus from each parent. These loci are represented by letter codes unique to each mutation of that gene. To record a gene “sequence”, we write out the loci of each of the genes in a row. Dominant genes are generally represented with a capital letter, and are often written before a recessive partner.
Because dominant genes hide, or “mask” their recessive partners, if a mouse is believed to be carrying a dominant locus, the other locus will be shown as a blank, to indicate that we are unsure of the second locus.
Here is an example of a gene sequence, taken from a common field mouse, also known as a “wild” mouse: A_ B_ C_ E_ D_ P_ ww. For each dominant gene, we have left a blank as we don’t know what the recessive loci are.
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1.3 How the Mouse Coat Works
At the most basic level, there are only two colours that make up the wide range of gorgeous fancy mice coats: yellow and black. All of the colours found in mice are the result of the distribution and intensity, or the absence, of these two colours.
The mouse coat is made up of an undercoat and guard hairs. These two coats can be long or short, wavy, or even different colours to each other. Most of the genes that mouse fanciers work with affect the guard coat.
Continue to 2. Fancy Mouse Genes »
