Okay. So now let's talk about the discovery of crossing over. I can very easily, you know, explain crossing over to you, because there are textbooks on this. We know and have known for a while that this concept exists. But if we go back way back when, they had no idea that this existed. So, I want to go through two important experiments that really confirmed to the scientific community that crossing over was a thing that happened. And because it's kind of weird. Right? You don't expect chromosomes to just kind of break off a piece of themselves and be like, hey, you can have this part of me, and I'll take your part, and we'll be all good. It's kind of an unusual thing. So it's actually brilliant how these scientists figured out that this happened. So the first study I want to talk about is between McClintock and Creighton, and they studied corn. And so what they did is they had a corn, and it was heterozygous for color, which here you can see, and starch. And the starch is dominant, so this is the allele for this. So the chromosomes for this plant were this. You have one chromosome here, you have the second one here. You can see that you have heterozygous for color and heterozygous for starch, but you can see that the dominant alleles are on different chromosomes. The c allele is here, and the starchy dominant allele is on a different chromosome. Now the reason that they did this is in order to be able to look at crossing over. So if they didn't do this, they would not be able to look at crossing over at all, because it would just look exactly; you wouldn't be able to tell. So they did this so that you could easily tell crossing over. Now, they also added a very interesting component onto these chromosomes. So on chromosome number 1, it had two special markers. It had this thing they call the knob, which I'm representing with this circle. And they had a piece of foreign chromosome, which they had translocated, meaning that they had just, like, broken it off and attached it from another chromosome, on the other end. And they used these markers to be able to follow crossing over. Right? Because if this segment crosses to this end, it's going to take the knob with it. Whereas if this segment crosses over, it's going to take this foreign chromosome with it and they can actually visualize it under a microscope. So what they did is they mated their chromosomes, or their organisms, this one here, with a colorless starchy plant, which is here. So what they found is that the offspring, some of them looked parental. So what do I mean by that? Well, it looked like this. It was heterozygous or it was dominant for color and starch. But some of them were recombinant, which means that it wasn't heterozygous dominant. It was heterozygous dominant for color, but homozygous recessive for starch. And that was very odd. Not necessarily how it's supposed to be. And so recombinants mean that it's a mixing between the two parents. And you can talk about recombinant genotypes and you can talk about recombinant phenotypes. But essentially, if you have a red tall plant and you mate it with a red short plant, then it's going to be recombinant if you mix those two together if you get short and red. So those are recombinant genotypes or phenotypes. Now, chromosomal markers were able to identify the recombinants because when they had crossed over, either the knob or the foreign chromosome came with them. So let's look at this. So here, this is what we started with. Right? This is the cross that we did, here we have our knob and our chromosome, and here's our mate. And so this is the offspring down here. And what you can see is that crossing over occurred because now you have a chromosome that has the foreign piece but no knob, and you have a chromosome that has the knob but no foreign piece. And they are also recombinant. Right? They don't neither one of these look like the parents. So here you have the here you have a colorless waxy, and this one was colored starchy. And here you have colored, colorless starchy, which actually did look like this. But this one didn't have the knob and this one did. So it actually looked different from the parenteral. So these are the recombinant types here. So this is a really important study, and you probably will be asked about this study in some way. But the important parts of this study are using these chromosomal markers, the knob and the foreign chromosome piece, in order to be able to identify that the chromosomes crossed over, they switched out parts of each other at some point during the development of these gametes. So with that, let's now turn the page.
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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Crossing Over and Recombinants practice set
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