Genetics is a fascinating subject that deals with how traits are passed on from generation to generation. Anyone working with rats can benefit from a general understanding of the study of inheritance for the appreciation of the many wonderful varieties that rats come in! Furthermore, an understanding of genetics will allow breeders to improve the health and longevity of their herd. Lastly, learning about coat color inheritance will make possible the preservation of a particular coloration and facilitate not only the development of something new, but also the ability to reproduce it! This page attempts to provide an explanation of genes, coat color genotypes, inheritance, and coat type inheritance.
Before I begin explaining the patterns of inheritance, let's have a quick chat about what DNA is and where it comes from.
Our body is made up of cells. There are two kinds of cells. There are body cells and sex cells. Almost every cell in our body, has DNA, stored in the nucleus.
The chromosomes are always stored in the nucleus of a cell.
A gene is therefore, a portion of your DNA. You receive two copies of genes. If a gene you received from your parent happens to be dominant, then it is said to be expressed and observed in the offspring.
If the offspring carries a recessive gene, then this trait will not be expressed or seen. It is said to be hidden, in the sense that it is being masked (not activated). However, in the absence of a dominant gene, like in the case where there are two copies of the recessive gene then, it will not be masked. So, if there are two copies of Russian blue as in the case of aa dd then, the rat will be or look Russian blue. Remember, if the rat carries even one copy of the dominant gene, for example aa Dd, then the rat will be black. The genes are converted into messages, called proteins. An example might be, a protein-enzyme that produces the pigment that colors the fur. The gene will influence the amount of pigment produced, leading to a diluted or full strength coat color. However, genes also affect health, growth, coat type, coat color, ear shape, to name a few. The genes that we will focus on in this particular tutorial deal with coat color, coat type, ear type, and body markings.
Rats are born with 42 chromosomes. This means that every rattie receives 21chromosomes from each parent. It also means that they have received two copies of every gene! If the copies of the genes found on the chromosome are the same; Let's use the albino rat as an example. They have a lowercase "c" on the c-locus which would look like this: "c/c" making the rat the white color that is has, it makes the rat homozygous at that particular locus. However, if the rat has two different types of alleles at any locus, the rat is said to be heterozygous.
Some of the traits that we are interested in, as breeders, are the resulting phenotypes that come about through the activation and interaction between the dominant recessive traits.
Interms of genotype language, an upper case "C" is considered dominant and the lower case "c" is therefore, recessive.
Have you ever looked at a genetics page for rats and see something like this, A_ DD? When you are trying to figure out what genes your rats carry there might be holes. The holes are the genes you don't know about. You might have an agouti rat but you are not sure if she carries a second copy of the Agouti gene (AA DD) or if she has one copy of the genotype. It is important to know this because having two copies of a gene will change the results you see by affecting color type and pigment intensity.
How can you tell if she is homozygous (AA) or heterozygous (Aa)? First, if you know what her parents where you can use this information to make an educated guess. If both parents were Agouti and the grand parents were all agouti then it is a safe bet that your female rat is also a homozygous at that allele making her (AA).
Sometimes, as we often discover, people make mistakes in identification and pedigrees are not always accurate. A second way to determine genotypes, is to perform a test mating. If you were to breed that agouti female (A_ DD) to a pure black male (aa DD), and all of the babies were Agouti, then it would be a safe bet that your agouti female is agouti.
Why? Click on this link and add the following genotypes (AA x aa). The (AA) is the mother, homozygous agouti, and the father is (aa), for black. The result you would see is that 100% of the offspring are heterozygous, meaning that they have one copy of the dominant gene (A), and one copy of the recessive version of the gene, (a). If the offspring where approximately 50% black and 50% agouti, then you would know that the mother has to carry one copy of the (a) recessive version of the gene.
Return to the genetics calculator to see for yourself. Use the following genotype for the mother: (Aa) and use (aa) for the father. This principal can be applied to most genes in rats. It works on traits that are governed by simple dominant and recessive genes.
Here is a chart that lists the genotypes and the coat color that would result. Think of each letter as a word in a sentence. The combination gives the sentence meaning. The meaning in this analogy, is color and type.
Very rarely will you find a rat that is exactly the combination you see below. They usually carry several dilutes, or recessive traits they inherited from their parents. You need to study a pedigree to determine what other dilutes your rats may have inherited. They will all have an effect on the possible outcome of a mating!
| Resulting Color|
Agouti AA BB CC DD GG MM PP RR
A true Agouti with dark rich color will have these genes in their dominant form.
The second genotype will also look agouti.
This is an example of a rat that carries what are called dilutes.
These alleles will alter the color of the coat. The dilutes are the lower case letters.
An agouti with dilutes Aa BB CC Dd Gg Mm PP RR
Blue Agouti Aa BB CC dd GG MM PP RR
This is the correct collection of alleles for a Russian blue agouti.
The dd allele will dilute the black to blue.
The second combination should produce a Russian Dove Agouti or Russian Cinnamon .
Aa BB CC dd GG mm PP RR (Russian Cinnamon)
Cinnamon Aa BB CC DD GG mm PP RR
Cinnamon is a mink based.
This means that when the gene on the M locus is present in
its recessive form, "m" brown agouti coat is diluted to a cinnamon.
Black aa BB CC DD GG MM PP RR
A true black will have recessive aa and the rest of the listed genes should be dominant.
When a black rat carries other recessive traits they tend to look dull with a
bluish coat or brownish tones.
While they are not desirable if you are breeding for the show ring,
they might be useful for breeding other colors because you know that they
carry at least one dominant gene for each of those traits!
This is a black carrying dilutes on several loci.
This is a black male that carries both the Russian Blue and American Blue dilutes.
Chocolate aa bb CC DD GG MM PP RR
The Chocolate is said to be deep rich and devoid of purple over tones.
Russian Blue aa BB CC dd GG MM PP RR
The Russian blue is a dark steel color.
There are different shades seen and this is most likely caused by various dilutes.
American Blue aa BB CC DD gg MM PP RR
To create this classic, you need to have a double recessive "gg".
I added the second genotype to show that with recessives you will have american blue,
however the "flavor" of the coat will change.
Recessives will change the overall hue-lightening and so on.
Here I am suggesting that you should try to work these out, through selection, to preserve the true color.
aa Bb CC Dd gg Mm PP RR
Silver aa BB CC DD gg MM pp RR
This is a subtle color.
One way to tell the difference between a silver and the platinum, is by looking at the eye.
The silver rat will have a pink eye.
It has been said that they are really just pink eyed blue.
Platinum aa BB CC DD gg mm PP RR
The platinum is a light gray color and is distinct from silver.
The platinum rat should possess ruby eyes.
The second genotype is that of an Agouti platinum.
Aa BB CC DD gg mm PP RR
Russian Dove aa BB CC dd GG mm PP RR
A lovely purple grey color.
It is based on both the "d" and "m" (mink) dilutes.
Russian Silver aa BB CC dd gg MM PP RR
Bright Silvery color with dark eyes.
Mink aa BB CC DD GG mm PP RR
A dark brown color with purpley tones.
It is based on a double recessive on the m locus.
Champagne aa BB CC DD GG MM pp RR
The color champagne is a subtle color.
It is a black based rat that has the double recessive on the p locus.
This gene dilutes the black hair to champagne.
Beige aa B* C* D* G* M* P* rr
The beige is a cream colored rat with dark eyes.
Fawn or Topaz Aa BB CC DD GG MM PP rr
It is an agouti based color that is diluted by the double recessive on the r locus.
Amber A* pp
This is an Agouti based color that is diluted by the double recessive p gene.
Albino AA BB cc DD GG MM PP RR
The albino may have any combination either the genotype on the top or on the bottom as
long as the "C" is recessive the rat will not malke any pigment and therefore be white with
If you find a rat that is white and has black eyes, then we are dealing with
a rat that has black eyes and a tremendous amount of white spotting, and not a
black eyed albino!
aa BB cc Dd Gg Mm PP RR
Pearl aa BB CC DD GG mm Pepe PP RR
The dark eyed whites in this litter are pearls. The orange/brown (lower left) is a cinnamon pearl and the
baby on the lower right is a smoked dark phase pearl.
Dark Phase UK Pearl aa mm Pepe RR with "smoke gene" which causes pigment to be deposited when hair is first being produced
then the pigment production is turned off and no pigment is produced and the hair is white underneath.
Ruby Eyed Pearl Berkshire Blaze (aa mm Pepe rr)
The pearl gene will only reveal itself in the presence of a double recessive on the m locus.
Pearl Merle (Meme)
Merling, is a spotting or tiger striping over the pearl coloration. The spots shape can vary in size and number.
Cinnamon Pearl Pearl Merle aa BB CC DD GG mm Pepe PP RR Meme
Himalayan/Siamese aa BB cch DD GG MM PP RR
The Himalayan rat is similar to the Siamese, however they tend to be white with lighter
brown points. The reason for this is due to the "c" which you may remember dilutes.
The "ch" gene, causes pigment (brown) to be produced at the extremities.
This is a picture of a blue point himalayan. They have a white coat, lighter points, lighter ears and eyes in comparison to a siamese.
Seal Point Siamese
BlackEyed Seal Point Siamese
aa chch BeBe
Blue Point Siamese chch dd GG MM pp RR
Also has dark points, but now we are dealing with two ch alleles and two recessive alleles for
Russian Blue. This creates a blue pigment that is produced at the extremities.
Burmese aa BB chch DD GG MM PP RR BuBu
This combination would produce a sable burmese with nice dark points.
aa BB cch DD GG MM PP RR Bubu
The picture above shows a burmese to the left and a mink to the right.
For comparison, the second combination would produce a burmese that is lighter in
body coloration and has light points.
Wheaten Burmese Aa BB chch DD GG MM PP RR Bubu
This would produce a wheaten that would have dark points and darker body coloration,
in combination with the agouti gene.
Blue Burmese aa gg chch mm Bu* The baby to the far right is an American blue burmese.
Cinnamon Wheaten Burmese A*D* chch mm Bu*
Black Eyed White
aa +(Extreme Spotting Gene)
Aa +(Extreme Spotting Gene)
AA +(Extreme Spotting Gene)
The BEW has an absence of color due to an extreme amount of spotting that is caused
by a gene or several genes. Not to be confused with the Be gene common to black eyed
siamese. The female pictured above is almost a perfect BEW. She did get some black coloration as she matured, as seen above.
aa bb gg
Havana or Mocha
This color will be different depending on what other recessive genes the line has or the rat has more specificly.
aa mm Rr
amo a* sdsd
a*a* mm sdsd
Black-Eyed Silver and Tortie
a*a* dd mm sdsd
In order to determine the genotype of the rats you wish to pair, you must think about the genes that each of them carry. This is one reason why it is so important to get your hands on a pedigree! I enjoy researching rat pedigrees and to try and construct an accurate genotype to help me determine the outcomes of a mating long before I ever pair a rat up!
This is important to me because I hope to achieve, improve on, and preserve the color types I am interested in, with as few matings as possible.
If you can, I would suggest going back at least three to four generations. Create a base genotype for the great grand parents. Then, looking at the grand parents, of the rat you wish to breed, use their phenotype and parents genotypes to construct their genotype using the information provided above. Continue in this manner until you reach the very rat that you wish to breed.
You can use this website to plug in a couple of genes and find out what the offspring may look like. This program spits out the outcomes for you but doesn't tell you what the combinations mean. That is what the chart above is for. You just have to match them up. You might find something completely new! If you have questions you can always email me! I would be happy to look at a pedigree or a combination and help you figure it out!
When a rat is said to carry dilutes, it means that they have copies of recessive alleles in their genotype. In other words, let's say you have a black rat that had one Russian Blue parent and one American blue parent. The genotype for a true pure Russian blue is aa dd. The genotype for a true and pure American Blue is aa gg. If you bred them together, you would end up with black kittens that had the following genotype: aa Dd Gg. These kittens, will not be as dark (black) as a rat that was a true and pure black (aa DD GG) because they have the recessive "g" and "d" in their genotype. These genes, do influence the intensity of the pigment that is produced in the coat color. If you cross these offspring, you should end up with black, Russian Silver, Russian blue and American blue. Although, this would be nice, unless you are sure about the dilutes carried by the parents (if they are not pure) you may have other surprises in store!
How might a Russian Blue that is paired up with a Black give birth to:
Rat A: Pedigree EVWD Pixie Dust (Black)
Rat B: Pedigree EVWD Rialto (Russian Blue)
Genotype of EVWD PIxie Dust
(aa BB CC Dd Gg Mm)
Genotype of EVWD Rialto
(aa BB CC dd Gg Mm)
The genes we enter into the punnett square calculator would be
EVWD Pixie: Dd Gg Mm
EVWD Rialto: dd Gg Mm
When these three genes are run through a genetics calculator, you get the following possible outcomes in one mating. Provided that there are not any other genes that will have an effect on the genes we selected you should expect to get...
In this section, I will briefly describe the variety of coat types found in the rat hobby, and their inheritance where known. Sources are found below or from experience.
|Coat Type (Trait)||Example|
|Smooth Coat |
The standard smooth coat is the wild-type form. It is smooth dense and the hairs lay flat against the rats body. There should be a softness to the coat when the rat is young and a slight coarseness that develops as the rat approaches old age.Lastly, the whiskers on a smooth coated rat are straight.
** It is interesting to note that some rexing genes that govern the amount of curl in the rex coat, may be present, however; masked; much in the same way that burmese is masked in the presence of "C". They might also be carried, and not expressed with one copy of the dominant form. This information is important when considering which coat type to pair up with a rat that has a "smooth" coat. It might be worth while to explore the pedigree to determine what type of, say for instance, rex-type genes may be present. They might have an influence on the appearance of the resulting litter, if rex or velveteen is desired.
The ever popular Rex coat, is the result of several different types of mutations to the standard smooth coat. Young ratlets possess a softer coat when young and develop a coarse wooly coat, once they approach adulthood. Rex have more hair "covearage" in comparison to wavy or double rex.
Rex x Velveteen Crosses
This is the outcome that might be seen when a rex and velveteen is crossed. There is a distinct mix between rex like coat (curl) with the softness of the velveteen.
Rex Sparse Coat
This coat starts out curly and after first or second molt becomes sparse, scraggily, and coarse. The hairs appear to be longer, most likely due to the fact that the hair is bent and not curly.
Coarse and Straight Rex Coat
This Rex-Like coat is thicker than the Sparse straight and does not possess bald patches. Their coat was semi-wavy at a young age, at first molt, they lose curl and as the coat comes in, it is slightly bent, coarse, and gives a puffed out appearance. The coat appears longer than the curled rex coat because the hair grows straighter.
The image above shows the type of curl to the whiskers on a rat that has this type of coat.
According to one source, the wavy coat is thin, and the individual hairs are said to possess several kinks or curls. The whiskers are also curly, bent and thin. This particular trait is different from the rex coat, as evidence by the fact that when bred to a rex the resulting litter will display a blending of both rex type as well as the wavy type coat. There are no health issues related to this coat type.
| Teddy Rex|
Teddy Rex Coat
The Teddy Rex is curly, thick and less coarse than the Rex coat. Whether it is it dominant over the Rex is unknown. It is possible that this coat type is a combination of known alleles or a new mutation.
Double Rex Coat
Depending on the alleles carried in any particular line, double rex, has a varying affect on the coat of a rat. When young, their coat develops similarly to the rex, however, at their first molt, they lose hair. A new coat will appear in its place, however thin. The cycle will continue to repeat itself with each molt. This female represents one example of a double rex. She has varying patches of thin hair.
Her coat is coarse and thin, with little curl.
Soft to the touch, longer guard hairs, wavy, not tight curl.
Short coat with a slight curl to the whiskers. The whiskers on a kitten that has the velveteen coat, start out very curly.
The Velveteen is distinctly soft, lacks curl, has kinks, and is shorter than the rex coat. The genetics of this coat are unknown. It is possible that the velveteen is a new mutation or a combination of two or more alleles influencing the coat's quality.
| Double Velveteen|
This kitten above, is a Beige Double Velveteen dumbo. This picture was taken when the kitten was around 14 days old. They have curly whiskers and the noticeable texture to their developing coat, in comparison to their "smooth coat" siblings.
Then, as their coat fills in, it becomes very curly and thick.
Right around week four, their hair falls out and they look like they are actually hairless! Then, within a week or so, it all comes back in! The way they finish out depends on several factors. I have seen double velveteens have this close cropped look (as pictured above) and others get the dense long curl again too! Others stay hairless.
Genetics are unknown at this time.
Long straight wispy hair. Gives the appearance of having a bad hair day. The Harley coat is recessive to the standard coat.
The coat appears to stand on end and gives the impression of a bad hair day!
Baby Harley coat and wavy whiskers
They have a greasy or shiny appearance to their coat and there is a unique molt that is seen in the harley coat type around 4-6 weeks. This coat type sheds out and then grows back in. There are several lines that possess both skin issues as well as aggression problems. However, several healthy Harley lines exist!
Curly/Wavy Coat Harley
Harley coat is recessive to the standard coat. The coat is
said to have a relaxed curl. Some people refer to it as the "rex-type" coat. Whiskers appear to be curlier on this type.
Satin is recessive to the standard smooth coat. The satin coat looses luster as they age.
(mink dilute smoked satin)
This coat is found to be smooth, shiny, and longer than standard short coat type. They also do not appear to have longer hair on the faces as kittens in comparison to Harley coat type. Here is a photo of a new born satin ratlet (upper) You can see the distinct direction of the whiskers in comparison to a smooth coat litter mate (lower).
The true hairless, the Sphinx, is lacking all body hair; however, there are different types of alleles that result in hairless rats, consequently, some hairless are found to possess hair on both the facial and hind regions. Hairless rats tend to have warm skin. Also, the whiskers on a hairless are very curly.
Ratlets that are hairless, have extreme curl to their whiskers. They also develop a a very fine coat and shed most of it away, depending on the line, between 4wks-6mos.