Rat Genetics

Copyright Laurie Hale, 2001. All rights reserved.

1. Each rat carries genes (alleles). They pass on one copy of each alleles type to their offspring. We'll talk mostly about colors, dilutions, ears and coat.

2. All rats are either
aa = black (non-agouti)

A/ = agouti

They carry these traits regardless of their phenotype (what they express, appearance).

3. Coloring is expressed due to the extra color genes, modifiers or dilutions the parents pass onto their offspring. These color genes are shown by letter symbols.
A capital letter such as A for agouti means it is a dominant color gene.
When you see A/- it means the other gene has not been identified as an A (agouti) or a (black). (phenotypes: Aa=agouti, AA=agouti, aa=black).

Two recessive alleles (expressed as a lower case letter here) on the same locus (location on a given chromosome) must be present for black-based ("self") colors, which are recessive to agouti (ticked; wild type), to show. When a animal inheirits 2 alleles, it is call homozygous (homo meaning same - 2 of the same) for that trait. When an animal only inheirts 1 gene, it is heterozygous, making it a carrier. Example = aa gg is a blue homozygote (a blue rat) while A- Gg is an agouti rat that is heterozygous (carries) for blue.

4. The presence of additional genes, such as dilutions, can change these basic colors.

Colors genes are shown as:
bb- chocolate
mm- mink
gg- American blue or slate
cc- albino

and all those colors will either be on a Black = aa or Agouti = A/- Base. All other colors are modified versions black or agouti. Below are the eye dilutions:

Red (Ruby) Eye Dilution or R.E.D. (rr)
Pink Eye Dilution P.E.D. (pp)

Agouti combined with R.E.D. (A/- rr) becomes fawn
Agouti combined with P.E.D. (A/- pp) becomes amber
A black rat combined with R.E.D. (aa rr) becomes a beige rat
A black rat combined with P.E.D. (aa pp) becomes a champagne rat

There are also color dilutions. Here are some examples of the Russian Blue dilution (dd) in action:
Dilution combined with black (aa dd) - Russian blue
Dilution combined with Am. blue (aa gg dd) - Russian Silver
Dilution combined with chocolate (aa bb dd) - Russian Dove
Dilution combined with mink (aa mm dd) - Russian mink
Dilution combined with R.E.D. (aa rr dd) - Russian platinum (aka blue-beige)
etc.

5. All these color genes work in pairs, one gene from each parent, and can be combined with others to produce different colors. See the Color Locus page for many combinations.

ie: a blue rat will genetically be this: aa gg
if we breed this rat to another blue rat (aa gg) we will get all blue babies (aa gg)
Now, if we breed this rat (aa gg) to a beige rat (aa rr) we will get all
black babies: (aa Gg Rr) These babies will carry blue and R.E.D. Only doubled recessive genes are expressed, so we see black (aa), but the rat only has 1 copy of the blue (g) and red (r) genes, therefore they are carried and not expressed.

If we were to breed two aa Gg Rr (black carrying blue and R.E.D.)'s together, what kind of combinations do you think we would get? Lets see:

The possibilities are: aa Gg Rr x aa Gg Rr = aa Gg Rr (like parents), aa gg Rr (blue carrying R.E.D.), aa Gg rr (beige carrying blue), aa gg rr (platinum), aa GG RR (black carrying neither blue nor beige).

You can figure out more of this type of probability for breeding by learning to use Punnett squares (see tables below).

RAT #1 is homozygous for Black and Russian Blue, heterozygous for dumbo, and homozygous for rexing.

RAT #2 is homozygous for black and heterozygous for Russian Blue.

What will the offspring look like?

Step 1 - Write out the genotype of each:

RAT #1: a/a B/B C/- d/d Du/du Re/Re

RAT #2: a/a B/B C/- D/d

Step 2 - Eliminate the common genes (shown in green above):

RAT #1: d/d Du/du Re/Re

RAT #2: D/d

Step 3 - Add in the hidden recessive genes (shown in orange below):

RAT #1: d/d Du/du Re/Re

RAT #2: D/d Du/Du re/re

Step 4 - Make Punnett Squares, one for each trait (in this case, D, Du and Re):

7. Hair
Rex (curly hair)
Rex is a simple dominat trait (meaning dominant over all other coat tyoes) and is shown by Re.
Rere is a rex
ReRe is a double rex, or Patchwork Hairless (a rat with random patches of hair on body).

rere is a standard haired rat.

*note- there are several different types of rex, this is the basic type.

Teddy Bear Rex/Velveteen (softer curl) is also dominant over standard, expressed as Cucu. CuCu is known as double rex. Most of their hair falls out and they can appear hairless.

True Hairless is a recessive trait. There are several genes that can cause hairlessness.
fz/fz = fuzzy
hr/hr = hairless

8. Husky & the Spotting Gene
Anyone choosing to breed any rat that carries the white spotting gene should be familiar with megacolon. Megacolon is a symptom of the white spotting gene forming at a poor time during fetal development. Megacolon (MC) forms during the fetal development of the neural crest. This interference happens at the time the nerves that serve the colon are developing. A kitten with MC will or may appear normal until weaning or starting solid foods. The symptoms develop between 3 and 6 weeks (average). Symptoms include bloating and intermittent diarrhea. There is no known treatment that stops it from happening.