Okay. So now let's talk about crossing over terminology. This is super important because when you're reading these word problems or you have questions on tests or whatever, they're going to be using special terminology to talk about, you know, which chromosomes and how things are interacting and crossing over. And you have to be able to understand the terminology to understand what's actually happening. So, you're going to have to memorize these words. But, hopefully, you know some of them already from previous classes, but crossing over occurs between two chromatids. Do you remember what a chromatid is? Well, chromatid, so tin, is going to be DNA plus protein. Chromatids are essentially just a single copy of one of the chromosomes. Okay? So, like I said, it occurs between two chromatids. Now you have sister chromatids and non-sister chromatids. And the difference is that sister chromatids are two copies of the same chromosome. Non-sister copies are copies, but they're not of the same chromosome; they're of the homologous pair. Right? So if we have two chromosomes, here they are. During meiosis, what happens is that they get replicated. Actually, let me draw these different colors and go back. So we have this chromosome and this chromosome, and these are called what? Homologous, I'm not going to write it out, but they're homologous pairs. Now, during meiosis, they get replicated. Sister chromatids of these, you can tell because they're the same color, whereas non-sister chromatids are going to be these. They're from the same original homologous pair, and they're both copies, but the non-sister chromatids are of the different homologous pairs, while the sister chromatids are of the same homologous pair of the same chromosome. So this is important. Now crossing over typically occurs in these non-sister chromatids because they're different chromosomes, you know, they're from the same pair but essentially different, but it can happen in sister chromatids as well. But generally when we're talking about crossing over, we're talking about non-sister chromatids. Now during meiosis, longest pairs line up and undergo crossing over. Now when I talk about lining up of chromosomes, what I'm talking about is actually the full four chromosomes that I showed before. So the replicated versions, they line up like this in the middle. So if I have another pair here, they're lined up in the middle of the cell during metaphase. Remember? Sort of refreshing your memory on meiosis, mitosis? Yeah. It'll come back. So, there are different terms to describe this. So these four here are called a tetrad. Dyads are pairs of two chromosomes or two chromatids. So that could be these two, could be a pair of sister or it could be a pair of non-sister. These are dyads. Generally, it'll work for is it a non-sister dyad or a sister dyad? Bivalent refers to the pair of homologous chromosomes. Right? So remember that these are two copies. So here we have, here we have the homologous pair. Bivalent refers to the pair. So here, this whole thing, when it pairs chiasmata. And thing. And then, chiasmata, and this is a structure that forms between two dyads, remember pair two chromosomes, or chromatids, are crossing over. Usually between non-sister, but can also happen between sister. So although I'm pretty sure this is a different language, chiasmata are here. So they are written here. So you can see that this, is a point where crossing over is occurring and here's another one, crossing over. So that's chiasmata. Now this is super important, here's another example of crossing over, you start with these chromosomes, crossing over happens, and you get the mixture that shows up. But, knowing these terms is super super super super super important. Now linked genes, remember genes on the same chromosome, also have certain terminologies. There's the cis conformation, and that means the dominant alleles of two genes are on the same chromosome. So if we're writing it like this, it would look like this. If we're writing it with the chromosomes, it's going to look like this, because the dominant alleles are on the same chromosome. The trans conformation, you can imagine, is exactly opposite. It means two different alleles or two genes are on the same chromosome. So you can write it like this, or if I'm writing this, it's going to look like this. So here you have a dominant and a recessive. And these are two different alleles, but they're on the same chromosome. This is cis, and this is trans. And again, I've mentioned this before, but linked genes are written differently. You can see it here. So instead of writing it like this, b, you write it like this. And this is because the alleles on the same homologue have no punctuation between them. And so, you can see this here, ab instead of, back up, ab. The slash is going to separate homologs, so it's going to look like this, ab, to represent this one up here, or if I wanted to do this one, it would be ab. That's ab, because this slash represents that they're on different chromosomes. And if linkage is unknown, which some of your questions will be, right, like, they're going to ask you they'll give you a bunch of information and ask, are these genes linked? They're not going to write it like this because that'll tell you for sure that they're linked. So what you're going to see is you're going to see a lot of this and especially this dot. This dot is used a lot to differentiate different genotypes if it's unknown if they're linked. So if you're answering a question saying, are these genes linked? Oftentimes, you're going to see it written like this with this dot. So understand the different ways to write things because you're going to be getting a lot of questions asked, you know, is it linked? Is it not? Is it independent assortment? Is it not? You know, is it all these different things or not? And each one of them is written differently. And so that can not only impact how you do the problem and understanding, you know, what you're starting with, but it also can impact your answer choice. Right? Because if you have an answer choice and one of them is written with linkage and one of them is written without, if you know for a fact is linkage, then that can help you figure out that problem. So, with that, let's now move on.
Table of contents
- 1. Introduction to Genetics51m
- 2. Mendel's Laws of Inheritance3h 37m
- 3. Extensions to Mendelian Inheritance2h 41m
- 4. Genetic Mapping and Linkage2h 28m
- 5. Genetics of Bacteria and Viruses1h 21m
- 6. Chromosomal Variation1h 48m
- 7. DNA and Chromosome Structure56m
- 8. DNA Replication1h 10m
- 9. Mitosis and Meiosis1h 34m
- 10. Transcription1h 0m
- 11. Translation58m
- 12. Gene Regulation in Prokaryotes1h 19m
- 13. Gene Regulation in Eukaryotes44m
- 14. Genetic Control of Development44m
- 15. Genomes and Genomics1h 50m
- 16. Transposable Elements47m
- 17. Mutation, Repair, and Recombination1h 6m
- 18. Molecular Genetic Tools19m
- 19. Cancer Genetics29m
- 20. Quantitative Genetics1h 26m
- 21. Population Genetics50m
- 22. Evolutionary Genetics29m
4. Genetic Mapping and Linkage
Crossing Over and Recombinants
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