Mendel and the Laws of Inheritance

Hi in this video, we're gonna be talking about Mendel and the principles of inheritance. So the first thing we need to talk about our alleles and I feel like people get confused when we talk about Aaliyah was just because they're not 100% sure what they are. So let's just define it very simply. And alleles are variants of a human gene. So one gene can have multiple alleles and each one of them are variants. And so because every human cell or every or every human somatic cell has um two copies of a gene, that means every human can have two alleles for that gene. And so we define combinations of these different in a couple of different ways. When we say we classify them as homo Ziegesar hetero side. So it's homos, I guess if the two allele copies that a person has are the same, they're identical and it's hetero cigarettes, if the two alleles are different. So if you have two of the same alleles will say that's homo zebras. If you have two different alleles, that's how it goes, I guess, but can also be classified in a different way as well, and that's a dominant or recessive. So dominant alleles are going to be if there, if you have a dominant allele, it's always going to be expressed no matter what the second is, it's always going to be expressed, you're going to see it in a typically you're going to see a physical appearance of it. Um but if those, if you have a dominant allele, it's always gonna be expressed, whereas recessive, alleles are different and it's only expressed pretty much if a dominant isn't there. So the dominant will always be expressed and so you'll see it if you have it. But if you don't have it then you will see a recessive allele. So what this looks like is we have this lovely animal here. I don't know what kind of animal it is. I guess you could make it up and um here are our two alleles for our different for our gene. And this gene is for um I guess coked color, right? And you can see there's one upper case and one lowercase here. And you're probably familiar with this notation from either high school or your undergraduate intro classes. But essentially this one is dominant heterocyclic because dominant means black. And we see this because it has the dominant allele, the upper case are. Now it's hetero ziggy's because the two alleles are different. You have one upper case and one lowercase. Whereas this animal for instance is different. It's recessive because there's no uppercase R. So instead you just have to lower case ours but it's also home as I guess because the two alleles are identical. They're exactly the same. So that's what those terms mean. Now I said we were gonna talk about Mendel and we are so Mendel studied alleles and gene inheritance and he did this by studying pea plants which I'm sure you know and he observed their offspring so there's a few different ways to label baiting and offspring that I want to go over. So the first one is called the P one generation. That's gonna be the parental plants. So those are the mom and dad plants. Then you have the F. one generation. So that's going to be the offspring created from the parental plants. So this is kind of like, you know, their Children, the plants, Children. But then you have the F. Two generation and this is the offspring created by the F. One. So these are the grandchildren, you have the parents, you have the Children and you have the grandchildren. Now in humans, we complete this analogy entirely makes sense. But what's interesting is that typically the F two generation can how we get that can be interesting and uses some kind of inbreeding we can think of from the F one generation. So we say it's their back cross if we mate the F. One offspring. So the Children with the parents and we think that's super weird for humans, right? It would be like so gross if that happened with humans. Um But in pea plants, it doesn't really matter, right because there's just some plants, so we can make the F. One generations with their siblings or with their parents to create these F two generations that can tell us interesting things about genetics. And it's not gross because it's plants. So um here's an example. So these are cats, for instance, just the generation. So you cross the two parents, you have the mom, mom cat and the dad cat. And they produce these four Children, these F one Children. And then in this case, I think the two of the f one Children are mated together. So these two siblings mate and they produce the f two grandchildren for the cats total inbreeding. But it's okay because we're studying genetics. So that's just some information hopefully will be helpful. So now let's move on.

Okay, so now let's talk about Mendel's laws. So we're going to talk only about two of Mendel's laws. The law of segregation and the law of independent assortment. And Mendel of course, discovered these laws because or wrote about these laws because he was studying P plants and was noticing all of these important laws about inheritance. And so all of this is going to be focused on how different genes are inherited. So the first law is the law of segregation. So what this means is that says that two alleles for the single gene and this is the key part here, Single gene separate during gamete formation. And so it sounds like a lot of like complex words, but it actually makes a lot of sense. So we know that we get one copy of gene from our mom and one copy of gene from our dad. And this means that we get one allele from our mom and we get one allele from our dad. So when the gametes are forming, so those eggs and sperm, we don't get two alleles from each parent, we only get one. So that means that the two alleles that your mom has for in you have to separate to form the eggs. And when we get the or when the eggs and the sperm unite, those two leal's unite at random. So there's not one egg destined for one sperm, it's just all sort of random. Any sperm can hit any egg and you get one random alil from each parent. So that's the law of segregation. That says that we don't get two copies of alleles from each parent because then we would end up with double the genetic information we need, we only get one copy of one allele because those separate during gamete formation. So during the formation of the egg or the sperm. So that's the simple one, the more complex one and the one we're going to spend a lot of time on is the law of independent assortment. So what this says is this is we're talking about different genes. So that's why I highlighted that single gene before this is really how you can tell these two apart. Um So it says that the alleles of different genes are passed independently of each other. So for p plants, what this means is that a pea plant of a certain color can be any shape. It's not that one color. A green pea plant has to be this shape. Those are two different genes. There's the gene for color and the gene for shape and any color Plant can be any shape because their past independently of each other. So in humans, that would mean that just because, you know, I'm 54, that my height has nothing to do with my hair colour. I can have blonde hair and I can be any height and because those are two different genes and therefore their past independently, and I can be any height and have still have blonde hair. They don't go together. And so when we're looking at this, we do I'm going to talk more about independent assortment. But first I want to go back to these crosses that you're probably familiar with. The mono hybrid. Cross is going to look at the inheritance of one trait. So whether or not I get blonde hair and the dye hybrid cross will look at the inheritance of two traits. So whether I get blonde hair and whether I'm certain height, an example of this. So we look at a di hybrid cross, you can see here that we're crossing this white cat and this brown cat and the white cat has a short tail. And the brown cat has a long tail. And so when you get when you make those and they produce offspring, you can see that this produces in this case all brown cats, a short tails. And then if you mate these two, so this one and this one, what you get is you get a mixture, you get some brown cats with short tails, you get some white cats with short tails, you get some brown cats with long tails. And some white cats with long tails. And these crosses, um pretty much just tell you what proportions of everything you'll get based on genetics. That's more for genetics class. So let's go back into the independent assortment because like I said before, it's a bit more complicated. So um what the law of independent assortment says that genes will um will assort independently. So they won't be passed together necessarily, But you may be thinking about this and you may not, but I'm going to propose the question is, what about genes on the same chromosome? Right. So we get this one chromosome from our mom and one chromosome for our dad. And each one of these chromosomes have multiple genes on that. So if that were true, that would if the law of independent assortment were true, that would mean that, you know, all these genes on this chromosome somehow have to be able to sort independently of each other and they can't they can segregate independently. And this is due to crossing over, which if you remember what crossing over is be familiar with it from my Asus. Um but if you haven't watched that video yet or you haven't gotten over that yet, um Pretty much what crossing over is is it's a process where genes on the same chromosome um can actually go to the other one. So those the alleles can sort of switch with each other. And so this allows for genes on the same chromosome to be passed independently of each other because those can cross over. And so there's a limitation though, crossing over only works if the genes are really far away from each other, genes that are close to each other will actually pretty much always be inherited together. So what this is called is genetic linkage and genetically quint measures how frequently genes are co inherited. So how often does my blonde hair result in me being 54. So how often those are co inherited and that determines the distance on the chromosome. So of course I would have to get a huge population. Usually this is not done in humans, this is done in fruit flies or other organisms, but essentially you get a big population and you say, okay, how many of these have blonde hair and a 54. And so if there's a lot, that means that there are pretty much called a linkage group, which means that these two genes, blond hair and heights are so close together that they never cross over independently, they're always passed on together. But if there's not a lot of blond hair and 54 people, so it's all sort of mixed in, then that means that those genes are farther apart from each other and therefore can be passed independently. So what this looks like is you have a linkage group. So you have gene one gene to and gene three and you can see that they are close together. So this would be something that would be inherited together. So if I were to rearrange these genes and I were to put them in this situation where you have gene one over here, Gene three over here, then these would be less likely to be co inherited. Whereas if they were right next to each other, they would be more likely to be co inherited. And the reason for this is what it is because of crossing over. And this is a process that mixes up the wheels in um mitosis. And so if crossing over were to happen here because these genes are so close to each other, it's likely that they would both have crossing over together so they would both cross over to the other chromosome. Whereas there here is if crossing over happen here because this is so far away, there would be no reason why crossing over would also just happened to happen here. So crossing over occurring in this really tight situation on genes that are right next to each other is what allows these genes to sort of always are very likely to be co inherited together. Um So that's sort of the caveat to the law of independent assortment, which says that you know, genes are all these genes are sorted independently. Which is true unless the jeans are really close together and just happen to always undergo crossing over all the time. So that's kind of the exception to that role. But with that let's now move on