Rat Genetics
Genetics is a fascinating subject that deals with how traits are passed on from generation togeneration. 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. Introduction To 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 bedominant, 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. 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. Coat Color GeneticsHere 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! 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 beheterozygous. 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. What does this mean? Test Mating to Fill in the Blanks!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. |
What are dilutes?
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!
Coat Types and their Inheritance
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.
** 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.