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Welcome to the Genetics Primer
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 p
ossible 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.
Basic Genetics
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.
This DNA molecule is complex and very long. It takes the shape of a spiral ladder. DNA coils up into a chromosome, to save space.
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 th
e 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) 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.
Genotypes and Phenotypes of Rats
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. In ter
ms of genotype language, an upper case "C" is considered dominant and the lower case "c" is therefore, recessive.
Sources:
Dilutes: What does it mean to be diluted?
When a rat is said to carry dilutes, it means that they have copies of recessive alleles in their genotype. In otherwords, 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!
Coat Color GeneticsLinks
- Rat Genotypes-AFRMA
- Genetics- Teaches you inheritance
- Genotypes Page (Genetics Tutorial)
- Hawthorn Rat Varieties Page- Phenotypes (How the rattie looks)
Coat Color Genetics
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
Pronounced (A-goo-tea) 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
| |
Black aa BB CC DD GG MM PP RR
This is a black male that carries both the Russian Blue and American Blue dilutes. | |
Chocolate aa bb CC DD GG MM PP RR
| |
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
The amazing American Blue! A lovely color. 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
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 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 | |
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
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 | |
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 pink eyes. 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
Pearl Merle
This image was kindly provided by Kat Lovings at Rat Dippity Rattery. aa BB CC DD GG mm Pepe PP RR Meme
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. Himalyan
Seal Point Siamese aa chch
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
| |
Blue Wheaten Burmese A* dd chch mm Bu* Picture coming soon! | |
Cinnamon Wheaten Burmese A*D* chch mm Bu*
| |
Black Eyed White
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.
| |
Classic Lilac aa bb gg | |
| Havana aa mm rr  | |
| Midnight Blue a*a* sdsd | |
| Tortoise Shell amo a* sdsd | |
| Caramel a*a* mm sdsd | |
| Black-Eyed Silver a*a* dd mm sdsd |
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.
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.
The Chocolate is said to be deep rich and devoid of purple over tones. 
This is a subtle color. One way to tell the difference between a silver and the platinum, is by looking at the eye. 
The platinum is a light gray color and is distinct from silver. 
Bright Silvery color with dark eyes. 
The color champagne is a subtle color. 
The beige is a cream colored rat with dark eyes.
This is a lovely color! 
The pearl gene will only reveal itself in the presence of a double recessive on the m locus. 
Merling, is a spotting over the pearl coloration. The spots can vary in size and number. 
This would produce a wheaten that would have dark points and darker body coloration, in combination with the agouti gene. 
aa (Extreme Spotting Gene) 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!
Example:
How might a Russian Blue that is paired up with a Black give birth to:
- Russian Blue
- Russian Silver
- American Blue
- Black
- Mink
Rat A: Pedigree EVWD Pixie Dust (Black)
- Mother Russian Blue (grandparents are American blue and Platinum)
- Father American Blue (grandparents are platinum and Russian Blue)
Rat B: Pedigree EVWD Rialto (Russian Blue)
- Mother Russian Blue (grandparents are Russian Blue)
- Father Platinum (grandparents are Black and 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
Possible Outcomes:
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...
- DdGgMM Black
- DdGGMm Black
- DdGGmm Mink
- DdGgMM Black
- DdGgMm Black
- DdGgmm Mink
- DdggMM American Blue
- DdggMm American Blue
- Ddggmm Platinum
- ddGGMM Russian Blue
- ddGGMm Russian Blue
- ddGGmm Russian Dove
- ddGgMM Russian Blue
- ddGgMm Russian Blue
- ddGgmm Russian Dove
- ddggMM Russian Silver
- ddggMm Russian Silver
- ddggmm Triple Dilute
Coat Types and Inheritance
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.
|
Rex Re re | Rex Coat 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.
Coat is shorter and lies flatter to the body than Teddy Rex. It also contains more curl in some lines. |
| Rex/Velveteen | 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.
This litter was the product of a velveteen mother and rex father. The kittens ranged in curliness as some were distinctly curly and others only wavy.
In this photo, the kitten above is going through a molt. As you can see, at 4.5 weeks, they lose their coat, and as it comes back in, the quality of the coat will change. Right now, the coat is short and lacks much curl or wave. It is extremely soft to the touch. This kitten is also 4.5 wks old and undergoing a molt that is less extreme than a litter mate. This kitten has more curl that is thicker. You cannot see through the coat to the skin. The molt has not left the coat with out curl. One of the things I plan to work on is a distinct velveteen line and rex line. Then perform a series of crosses. I would like to determine the inheritance pattern between rex and velveteen.
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Re-Sparse Straight | 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.
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Rex-Type Coarse Straight Thick | 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.
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Wavy wv | Wavy 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 | 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. |
| Velveteen | Velveteen Coat
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 | Double Velveteen pic coming soon. |
| Velour | Velour Coat Genetics are unknown at this time. |
| Harley-Type A | Harley Coat 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! According DCR and T&MCC ratteries, a harley is easily distinguished from other litter mates. They have a greasy or shiny appearance to their coat.
Robin, from DCR, commented on 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! |
| Harley-Type B | 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 | Satin Coat Satin is recessive to the standard smooth coat. 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.
. Photo credit:RaffinHouse Rattery This is an adult standard satin Russian Blue Doe. |
Hairless hr |
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. |
Sources:
The Satin and Harley pictures and information kindly supplied by, Deer Creek Rattery.
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