Okay. So now let's talk about twin studies. And the reason we talk about twin studies is the fact that humans can't be bred. Right? We can't be bred to determine in different environments and with different genetics to be able to determine what's genetic variation, what's environmental variation, what's additive genetic variation, dominance genetic variation. So how do we actually have to do this? We can't set up these control experiments. So what we do is we use twin studies. And twin studies are taking twins that are produced and studying them for genetic or environmental variation. So there are 2 types of twins. There are monozygotic twins, and how this happens is there's a single zygote. So there's a single sperm that fertilizes a single egg. But what happens is that it does this weird division thing. It mitotically divides and splits into 2 cells. And so when it does that, there are now 2 babies that are produced from these two cells, but they're genetically identical. So they have the same genetics. So, therefore, when you study them, all the variation that they have is environmental variation. That's kind of why, if you look at identical twins when they are babies, they look very similar. Right? Because they haven't had a lot of exposure to the environment. But if you look at those same twins and then they're in their seventies, they actually can look quite different even though they're identical. And that's because that environmental variation has built up throughout the years to make changes in, you know, potentially epigenetics or made them more likely to get a disease or various things, right, that impact phenotype. And so, monozygotic twin studies are extremely important in being able to identify how much environmental variation plays on human genetics. However, it's not ideal, and there's actually some genetic changes that occur very early in development. So an example of this is copy number variances can actually change a lot, in very early development and this is a way that monozygotic twins, which would otherwise be genetically identical, may actually not be 100 percent genetically identical because each twin could come, could actually carry different numbers of copy variants. And there have actually been cases of monozygotic twins who are genetically identical pretty much of one twin actually developing a genetic disease and the other one not, very early in their life because they had variations in copy numbers. So then it's not a perfect system, but it's really the best we have. The second type of twin is dizygotic twins. And this is actually 2 fertilizations. So there are 2 eggs and 2 sperm. And, these are, although they are twins, they are just as close genetically as any other sibling. So if you have a sibling that isn't a twin, they are just as close genetically to you as a fraternal or dizygotic twin. But because they often share the environment. Right? They are raised in a similar situation. We can kind of use that to, at least in part, study genetics. It's not perfect, obviously. The monozygotic twin is definitely the gold standard. But we can also use dizygotic twins, especially ones that have been separated and raised in different environments, to study various components of human genetic variation. And so, an interesting part of expression, of twin expression, is that sometimes traits aren't expressed equally. And so a concordant trait is a trait that both twins express either way. So they either both express it, or they both do not express it. The second type is discordant, and that is when one twin expresses a trait and the other doesn't. And this can happen in both monozygotic and dizygotic twins. And so just some fun vocab for you, as we finish out this topic. 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
20. Quantitative Genetics
Heritability
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