Appearance and Genes

Phenotype Mutation Traits

The term genotype refers to the actual genetic makeup of a rat. Phenotype refers to the appearance of the rat regardless of the genes involved. they are not always the same.

For instance a white rat that is genetically an albino may look just like a pink eyed white rat that is not albino, but whose color has been faded out from dilutions. Although they both have the same phenotype (pink eyed white) they have very different genotypes (genetic makeup).

In the wild, rats are typically agouti (with an occasional albino or non-agouti showing up as a natural mutation. Their fur and whiskers are straight, the ears are placed on the top of their heads, and they usually do not have any white markings.

Looking at the rats we have in the fancy now gives you an idea of just how many phenotypical mutations have naturally occurred in the breed . They include any types other than agouti self with standard coat and standard ears.

Often, a mutation is not as simple as dominant or recessive. There are also mutated genes that act as modifiers or in combination with other genes, and within gene mutations there are usually variations and degrees of expression.

As mutations that affect phenotype have occurred and came to the attention of breeders, there have been some who took an interest in them and chose to work with them.

When working with a possible new type, a breeder will systematically breed, and inbreed the rats in order to learn.

Determinations involving Possible New Mutations
(The questions a breeder seeks to answer)

  • Is the trait caused by a genetic mutation?
  • Can the mutation can be reproduced?
  • Is the mutation harmful to the animals in any way?
  • Is the mutation of interest to the fancy?

Work on a new type that might be a benefit
to the fancy would be pursued while one that would be a detriment may
be discontinued.

Basic genetic mutations that affect Appearance (phenotype)

Coat/Eye Color

The different colors in the fancy today are mutations that affect the appearance of the original wild type which is agouti.

There is a recessive mutation on the agouti locus for non-agouti, and a non-agouti rat with no color dilutions is a black rat.

There are also color dilutions that can change the appearance of an agouti or a
non-agouti-based rat, and these color mutations are typically recessive. They can affect both coat and eye colors and include fawn/beige, champagne/amber, mink/cinnamon, chocolate, blue and Russian blue.

Color dilutes can be combined on an animal to make other shades.

Coat Type

In some lines, straight fur type mutated to rex or curly fur. It can present in different ways, and there have been multiple rex mutations in the labs, but the rex/curly type fur in the fancy is a dominant trait.

The fur can appear sparse or or even be nearly hairless when the gene is double dosed on an animal.

There are also several hairless mutations. Hairless mutations are carried recessively.

Genetically hairless rats can actually have various amounts of fur on their bodies and still be true type hairless.Their whiskers are curly from birth (as are rex).

Sometimes hairlessness is associated with certain issues such as:
inability to lactate, autoimmune disorders, predisposition to cancer and
skin problems.

Satin is another coat mutation. It is a recessive gene that produces rats with longer, shiny fur as well as whiskers that are slightly bent. The satin mutation also tends to add a yellowish cast to white or light-colored fur.

Ear Type

Standard ears mutated into a different type of ear called “dumbo” ears. Dumbo is a recessive trait that produces a rounder ear set lower on the head. There are many variations involving ear shape, ear thickness and ear placement.


Marking and spotting gene mutations add white to the normally unmarked (self) rat. Generally markings are genetically safe with the exception of “high white” markings, chinchilla, or dominant blazing which can be associated with aganglionic megacolon


Patterns include albino (and its allele, color point, also known as Siamese or Himalayan) which mask the color of the rat’s fur and eyes or only let a certain amount of color to show through in selected areas (as seen in Siamese). Another pattern is merle, spotting that shows on top of the rat’s color.


Tailless rats (referred to as “manx” by some) are missing all or part of their tail. The result can range from no tail at all, to a stumpy tail, to a shorter tail. Taillessness can sometimes be associated with severe birth defects.


Type includes characteristics such as body shape, size, build, eye set, ear set, tail length and head structure. These traits are often polygenetic.

  • Brooks, E. (n.d.). Basic Genetics of Color in Rats. Retrieved December 18, 2008, from
  • Hanson, A. (n.d.). Genetics of Coat Types. Retrieved December 18, 2008, from
  • Hanson, A. (n.d.). Mutations in Rat Coat Color. Retrieved December 18, 2008, from


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