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.