Basic Genetics of Color in the Rat Fancy

By Elisabeth Brooks, © 2005 Spoiled Ratten Rattery

This is a refresher on genetics from back in school as well as the basics of color genetics in rats. It is only a start, but it will go a long way in explaining what breeders are talking about when they start into color codes.

A Blank Palette, The Defaults

Before any color dilutes are "added" to a rat, there is the type they start with. Wild rats are "agouti" which means that they have bands of different colors on each hair. In the fancy, we also call the overall color that agouti with no dilutes looks like, "agouti" (RSA standard for agouti-golden chestnut brown with dark gray to black base fur and black guard hair, black eyes). The gene for agouti is "A," and it is dominant meaning a rat only needs one copy in order to display that trait.

There are rats without the wild type gene, rats that do not have the bands of different colors on each hair. These rats are called "non-agouti," and a rat that is a non-agouti without any color dilutes looks black (RSA standard for black-to be a deep solid black, devoid of browning or rusting, black eyes).

When determining the color of a rat, the most basic piece of information to obtain is whether the rat is agouti-based or non-agouti-based as they are all either one or the other. Then, you can narrow down which color dilutes have been added. If you do not know what the possibilities are either by visual inspection of the rat in question's parents or by looking at his pedigree for information about them, then you might be able to tell by looking at him. Part the hair on his back and look closely at individual hairs to see if you can see bands of different colors on them. Cream at the base of the hairs does not really enter into it as many non-agouti rats, especially blue or American mink-based colors, have a light-colored undercoat. Also, the belly of an agouti-based rat will be lighter than the rest of the rat and sort of silvery. That difference in shading/color will be especially noticeable on darker colored rats. After all this, it may not be possible to tell the difference between agouti-based and non-agouti-based on very light-colored rats including those with many color dilutes or on white rats without breeding them.

So "agouti" refers to both the type, a base, as well as the color. "Non-agouti" is the type/base, and its corresponding color is "black."

Thinking Inside the Squares

The genes for color (and for pretty much everything else) are inherited from a rat's parents. They have 42 chromosomes, and on each chromosome there are various pairs of genes, each on their own locus. These genes affect all sorts of characteristics of the rat including health, temperament and physical appearance. The genes are in pairs because they inherit one from each parent.

Previously, the "default" types, agouti and non-agouti were covered. The symbol for agouti is "A." Again, agouti is dominant meaning a rat only has to have one gene for agouti in order for that rat to have the bands of different colors on her hairs. If the rat has one copy of a gene, in this case agouti (A/a), she is said to be "heterozygous" for that trait/gene. If she has two of the same gene, again in this case agouti (A/A), she is said to be "homozygous" for that trait/gene.

Going back to high school biology, let us use a Punnet Square to show the results for the agouti locus if we breed two heterozygous agouti rats together.

The projected outcome of the litter is 25% non-agouti and 75% agouti. One third of the agouti rats will be homozygous. The breeder, however, will not be able to tell the heterozygous agouti from the homozygous just by looking at them. To find out about a particular one, the rat would have to be bred to a non-agouti.

Adding Color Dilutes

Most of the dilutes in the fancy that affect color are recessive. Recessive means that they are not expressed, you cannot see them on the animal, unless that animal has inherited two copies, one from each parent. Therefore, an agouti or a non-agouti might "carry" color dilutes but not display them because they are just carriers; they just have one copy. A bit on dilutes-color dilutes are genes that cause the pigment in the hairs to dilute to another color or that cause them to clump in such a way that they appear to be a different color. Without color dilutes added (or "diluting") the color of the rats, they are the colors the fancy refers to as agouti or black. If a rat were to also have two copies of a particular dilute, such as red-eye dilute ("r"), then that rat would not look like what we call an "agouti" or a "black." If those two copies of red-eye dilute gene (often abbreviated as "RED" in conversation) are on an agouti-based rat (A/- r/r), then the color that rat displays is called "fawn" (RSA standard for fawn-to be a rich, golden fawn evenly ticked with silver guard hairs. Undercolor to be pale blue/gray carried down to the skin. Belly fur to be silver, dark ruby eyes). If the two copies of red-eye dilute are on a non-agouti base (a/a r/r), then the color is called "beige" (RSA standard for beige-a warm tan, dark ruby eyes).

Mink on an agouti-based rat is called "cinnamon," and on a non-agouti it is called "mink," etc. Pink-eye dilute on an agouti-base is called "amber," and on a non-agouti, it is "champagne," etcetera.

Combining Color Dilutes

Dilutes can be combined to make other colors. Blue and Russian blue combine on a non-agouti to make what is called "Russian silver." Blue and mink combine to make platinum or platinum agouti. The Punnet Square can be particularly helpful when a breeder needs to predict the outcome of a litter when it comes to color, markings and other physical characteristics even when there are multiple traits involved.

Clubs have not necessarily come up with a name for every gene or color combination on each base. Breeders will need to learn the standards of their own club and others to determine what colors are called, which are standardized and what they should look like according to their club as there are differences between countries and even between clubs within the same country.

This and That:

A color pattern is a color, but it is not a solid color like blue or amber or chocolate. Siamese is a color pattern caused by points of color showing through on an albino rat. Himalayan is the same way, but not as much color comes through because the rat only has one gene for color point. Merling is a pattern over a color such as mink or pearl. They are not markings, and they do not make a rat a tri-color. Markings are white spotting on the solid color of the rat and are caused by different genes.

Not all pearl rats die. "Homozygous lethal" means that when and if two pearl rats are bred together, the ones that happen to inherit the (dominant) pearl gene (Pe) from both parents will die, approximately 25% of the litter, but it will happen in-utero. In addition, the pearl gene only expresses on mink or American mink, though it has been suggested that it might also express on chocolate. A rat can be a pearl but not display it. A black rat can be a pearl (Pe/pe) and just not have the mink genes so that the pearl gene can display.

There is no "l" gene, yet, but then many of the gene designations in the fancy are hypothetical. They have not been studied or mapped, yet, and it could be a few years until they are mapped. Breeders need something to use to represent what they know exists, and they need to have designations that are uniform from one place to another, so that breeders are speaking the same language. The "l" gene (recessive, for "lilac") is a US hypothetical. It is not widely used, but it refers to whatever the mystery gene or allele is that lightens American mink to the color "lilac" in the United States. The same gene is suspected of being the one that lightens blue to "sky" or "powder" blue and sometimes even gives blues ruby eyes instead of black. If it exists, it is possibly linked to blue and/or American mink which could give scientists a hint as to where they are located.

The gene for silvering in the US (recessive, no designation that author is aware of) is, according to club standards, supposed to make 50% of the guard hairs on a rat silver. It is not supposed to make the other hairs silver. That can be selected for or against, but it does not, at this time, appear to be caused by a specific gene.

Last updated 11/2005

"Basic Genetics of Color" article and photos © 2005, E. Brooks of Spoiled Ratten Rattery. All rights reserved.