Basic Genetic Theories - Mendel Genetics

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!

Understanding Genetics - Reproduction

There are so many things to learn when you begin to understand genetics, not only for rats, but for anything. Yes, we understand that we inherit traits in our DNA from our parents, but how? That is today's topic!

Chromosomes
So... what are chromosomes??? Chromosomes, in a very simplified explanation, are tightly wound chunks of DNA. In this image you can see that there are pairs, also know as homologous pairs. In pair 1, one of the chromosomes some from the mother, the other from the father. It is the same for each of the following pairs. One thing to remember is that even though these chromosomes come from different beings (mother and father), as long as they are within the same species, the same genetic information should be present at the same place on each of the individuals in a pair. If the gene for, say, having a widows peak or not, were found on chromosome one, right at the top end on moms chromosome, the gene should be at the same spot on dads also.

As humans, we have 46 chromosomes that are grouped in 23 pairs. One of each of the chromosomes in a pair come from each of our parents. Essentially, half of our DNA comes from each parent. The same is true for rats. The difference is that they have 42 chromosomes, 21 pairs. Every bit of how that rat will turn out is programmed on those tiny clumps of protein and nucleic acid, the building blocks of DNA (and RNA, but that is a different story).

Our Sex Cells - Eggs and Sperm
The next thing to have a basic grasp of is how these genes all get passed to offspring, through sex cells, also called gametes. I will run through sperm cell production as it is slightly less complicated and goes through mostly the same processes.

Stem cells, located in the semeniferous vesicles (you can just know that this is in the testicles), are the origin for sperm cells. They have all of the same genes that we have present in each of our body cells. When it is time for that stem cell to become sperm, it undergoes something called spermatogenesis (Sperm making). First, there is a period where each of the chromosomes undergo a duplication phase, so each of those chromosomes that you saw up top make an identical twin attached to it. This is also known as Sister Chromatids.

 This image, showing what is known as Meiosis (cell division that makes sex cells), walks through the basic steps development from a stem cell to sex cells (but only using 2 homologous pairs). In the first cell you see 2 pairs of chromosomes. There is a red (from mom maybe) and a blue (from dad possibly) of each. After going though the duplication period you get to the second cell. Note that there are stuck together identical copies of each of the chromosomes. The following cell lines them up in preparation for the first division. The fourth cell, under B, shows something called crossing over, which is simply a means of genetic variation. This is ONE way that a parent is able to have 20 children and no two be alike unless they are paternal twins. C shows the first division. One of the long duplicated chromosomes went to each cell, and one of the short duplicated chromosomes went to each cell. The last stage, that gives you your gametes, pulls those duplicated chromosomes apart and separates them into two different cells. This means that each gamete, or sex cell, ends up with HALF as many chromosomes as the original stem cell.

Why is this??? This is because this is for one sperm. And ovum (egg) forms in a similar manner, and when they combine, you then have the full number of chromosomes and all of the needed genetic information in the offspring. I hope that provides a general understanding that will be the foundation for all of the future blogs in this series! If there are questions, please comment and I will do my best to answer them.