Siamese Rat Genetics
From the May/June 1997 Rat & Mouse Gazette
BASIC GENETIC NOTES
Here are some genetic formulae which may make the following comments clearer. A hyphen, -, means that any recessive gene can be paired, without much difference in color. All loci (this is plural for 'locus') have a pair of alleles (genes) at each site. The usual loci for colors in our fancy are these sites: A -, B -, C -, D -, P -, (and other sites for markings and coat quality).
The basic genetic loci we are dealing with in THIS discussion are: A -, C -, P - , so I will notate only these, in the following discussion.
BREEDING FOR THE SIAMESE TYPE
To start with, Siamese is only paired with the self-color gene, "a": if you combine it with the dominant "agouti" gene, "A", you will not get a Siamese. So, no matter what you have, if the first locus has the agouti gene, "A", you won't have Siamese, just a washed-out agouti color.
The "himalayan" gene responsible for the Siamese/Himalayan types, is at the "C" locus & is written as a small c with an "h" subscript, "ch" and is recessive to "C", so you need two doses of it (homozygous), to have the Siamese color show up, even with the "aa" locus:
What does all this tell us? If you breed Siamese x Siamese, you will get more Siamese (and if they carry albino, you will get himalayans and albinos, eventually).
If you need an outcross, get the darkest Black rats you can find. If they have white on their feet, you are in trouble, but do the best you can.
Since the himalayan gene is recessive to C, if you breed Siamese x Black, all the f1 generation (first generation litters) will carry one dose of Siamese at that locus, and will look black. Assuming that your black rats are pure, or carry no other recessive surprise genes, their genotype will look like this:
aa, C ch, PP ( i.e. self Black color, one dose of himalayan, and black eyes)
If you breed f1 kids back to the Siamese parent, basic genetics will tell you that, statistically, of the f2 progeny will be Siamese, and all the others will be blacks which carry the Siamese gene. If both parents are completely unrelated, you can breed the siblings together, which will give you a chance of getting Siamese rats ( 1/4 of the litter, statistically).
One thing to remember about this f2 progeny: the Black rats might be homozygous or heterozygous Black rats, so if you choose to breed these Blacks, breed to a Siamese if you can. Then, even if all the f3 from this are black, at least you can be sure that they carry Siamese.
If you choose to breed Black f2 x Black f2 rats, and get no Siamese, you won't be able to tell if these parent f2 rats carry the himalayan gene at all, so you will not know if their Black f3 kids who look great in other respects, even carry the himalayan gene.... they may be homozygous Black rats. You will also be breeding very closely-related animals... not a good choice for novice breeders.
PRACTICAL BREEDING TIPS
With the f1 generation, choose on the basis of color and size/type. The most prominent feature of the Siamese is, of course, the darkness of shading and points. If the Blacks have white feet, then their offspring will have this same white: whether Siamese or Black. If this is all the stock you have, at least choose the ones which have the best size, type, and the least white on the feet (I am assuming there is no white anywhere else, like on the tail, belly, etc. otherwise you are really working with a deck stacked against you!). Breed with only the best stock you can get, and from the best of your litters.
Keep in mind that Siamese color & points are temperature sensitive, so winter coats are always darker than coats which come out during warmer weather. If you are unsure of whom to breed on this quality, don't fault minor variations between litters until everyone has been through a moult in the same temperature.
The genetics of the Siamese mouse are essentially the same as for the rat. So...
As with mice, it is useless to carry ANY modifying genes at other loci which will dilute the color, such as PE, dilute (blue), mink, RE (fawn/beige), or of course, the albino at the same locus, which will just give you himalayans. Himalayan breeding will be covered in another article.