Medel Genetics
The person who first theorized much of we now know about simple heredity was an Austrian monk named Gregor Mendel. When you see reference to Mendel Genetics, Mendelian Genetics, or something similar, this is whom they are referring to. He found the basics regarding inherited traits through plants, but, the same things apply to most of the organisms who reproduce sexually (plants, insects, animals, etc). Keep in mind that his theories were much more advanced than what I am going to dive into but much simpler than that which a geneticist references. For our purposes though, it can be sufficient. One thing to understand that a "gene" is a unit of inheritance... It is a section of DNA that is passed to offspring.
First, a term that you should be familiar with, is Allele. An Allele is a letter reference to a specific gene. If you have looked at rat genetics in terms of genes, I'm sure you have seen that the first letters you see are aa, Aa, or AA. These 'A's are alleles. As I mentioned in the previous Blog, you inherit one copy of a gene from your mother, and one from your father, so that means you will usually have two alleles, one paternal, one maternal. The gene that it represents is called a Locus. (No... not the crop eating locust. LOL) 'A' is the Agouti locus.
The next terms to have a basic understanding of are 'Dominant' and "Recessive'. There are variations of these, but for now, simple is better.
A DOMINANT trait means that, if a rat possesses one copy of the dominant gene out of the "usually" two alleles possible, that trait will be seen in the rat. A dominant trait is written using capital letters when the gene is present, and lower case letters when the gene is not present. Take the gene for the Rex coat. This is 'Re'. That means that if you have a "Rex" rat, it's alleles under the Rex locus will be 'Rere' ('Re' for the dominant trait that you are seeing, 're' for the one inherited from the other parent)
A RECESSIVE trait isn't exactly the opposite, but it is easy to understand in the same terms. A recessive trait can only be seen if two copies are present. This means that one copy needs to come from each parent. They can go undisplayed in the parent and still be passed on to the offspring. Recessive traits are written using lower case letters when the gene is present, and capital letters when the gene is not present. In rats, many of our color genes are recessive. We can use "Mink" as an example in this case. The locus for mink is "M". For a rat to display the Mink color dilution, the two alleles within this locus must be 'mm'. ('Mm' means that the rat carries the gene, the capital 'M' means that gene is not present, the lower case 'm' means that the gene is present. It does not show because TWO copies are not present. This means that 'MM' does not carry mink.)
Mink babies
Photo Courtesy of Cathy Wingo
You may often see the terms Homozygous and Heterozygous. Homozygous means that the two alleles are the same, both present or not present (aa -or- MM). Homozygous Black rat will be 'aa' or "black". Heterozygous means that the alleles are not the same, one present, one not. Heterozygous Rex would be 'Rere' or "Rex".
Two other terms that you may see with some frequency are, Phenotype and Genotype. They sound pretty similar, right? They are, and they aren't. A Genotype describes the genes that something has. It included every carried trait in that organism. A part of a rats Genotype includes its carried, not expressed recessive genes. A Phenotype is the way the animal looks. A Mink (aamm) rat is an example of a phenotype, but that rat may carry Russian Blue (aammDd) which is it's genotype, regardless of the fact that the blue isn't expressed.
The next entry will teach a few of the known color Loci found in the rat genome, as well as beginning on how to use a Punnett Square to help you to determine expected offspring from rats. Stay Tuned!
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