Sex-Linked Genes - Video Tutorials & Practice Problems

Sex-Linked Genes

Hi in this video, we're gonna be talking about sex linked genes. So humans have two sex chromosomes, the X. And the Y. And um these are super important. They all they have different functions. They act differently and combinations of these determine whether or not we are female or male. And so the Y chromosome has certain characteristics. One is that it's actually a very small chromosome and it actually contains only a few dozen genes. So it's really in terms of genes, definitely the smallest chromosome that humans have. There's one particular gene on there called the S. R. Y. Gene. And this gene is what determines whether or not your mail and it is the male ness factor. This is the gene that all males have and it makes the males the so females X chromosome doesn't have this. So females do not have this gene. Now the Y chromosome is given a special term. It's called him eazy Vegas. And this is because there's only one Y chromosome. So if you think of all the other chromosomes, they come in homologous pair. So there's a copy of them. But in males there is only one Y chromosome, there's not a copy of it. It's paired with an X chromosome. Which means that it is easier because there's only one of them in an organism. Now the X chromosome is different than the why? Because the X chromosome is first much bigger. It contains hundreds of genes and these genes have multiple non sexual functions. So like the Y really only determines sex and determines male or not. The X chromosome is more important. It's a lot a lot more functions. And that's because the X chromosome is found in both men and women. So I'll show this below, but this is female, X. X. And X. Y. Is male. And so um both both sexes have an X chromosome. And so that's why the X. Has a lot more genes for non sexual functions because everyone will get them. Whereas the Y. Is pretty much just the sex determining chromosome, but they do the X. And the Y. Even though they're not the same chromosome, they have very different regions on the very different genes. They do contain one or actually two regions that are that can work together. So these are called pseudo autism, A regions one and two. And so these are copies essentially ones in the eggs and ones in the Y. And the purpose of these is to help pair them together so that during mitosis, those chromosomes can separate into gametes properly. So normally the homologous pairs have their their copies of each other, so they have things that will allow them to attach together and match up and then separate into the gametes appropriately. So you don't get extra and you don't get too little. The X and Y are two different chromosomes so they don't have that just inherently, you know, they're just exact copies of each other because they're not. So they have to have special regions called the pseudo autism. All regions one and two that are copies of each other that allow them to pair and separate so that you get the appropriate amount of excess and the appropriate amount of wise instead of, you know, too many or too little. That's what those regions are for. And so they can act as a pair and segregates equally into sperm or if in the case of X. X. Into eggs. Now obviously this doesn't go as planned all of the time. And there's a term called non disjunction and this occurs when chromosomes failed to separate properly. Now you can see this in homologous pairs. But this example here showing you in terms of sex chromosomes. So you can see that some organisms can end up with three exes or two Xs and A. Y. And then others can end up with just one X. Or one Y. And obviously none of these are desired because you either want X. X. Or X. Y. You don't want this like three combination or just the one. And we'll go over these um what these these are actually disorders and what these disorders are called in another time. But here's an example of the chromosomes. You can see the X chromosome, the Y. Is much shorter and this would be what a male or female. Right? This would be a male and the two exes would therefore be a female. So sex linkage is a term used to describe when the genes located on a sex chromosome have certain inheritance patterns, right? Because if there is a mutation or something on the X chromosome that's going to affect males and females differently because females have a chance of getting a normal X. Chromosome where males only have one X. And so they'll always get the mutant. So there's different types of sex language. The one I just described is called X. Linkage and there's gonna be mutants on the X. And that's going to cause them to inherit differently. There's why linkage, this is actually fairly rare. The reason is because the Y. Has many less genes on the X. But there is why linkage and that is mutant alleles in the Y. So only males would get this and then you have a couple of other sort of more nuanced terms. And these are sex limited inheritance. And this is when expression of a phenotype. So some kind of trait is absolutely limited to one sex. So, an example of this is in certain animal populations, um size or color is very different between the males and the females, for instance, the orb weaver spiders. I don't know if you've ever googled them or seen them or anything but orb weavers, the females are huge. I mean they're huge. They're like scary spiders that you burned down your house if you see them, but the males are actually tiny, you may actually miss them. And so that's a that's an example of sex limited because the males will always be tiny. Um And that is a phenotype that's absolutely limited to the male orb weaver spiders. Whereas the big the big, huge, like monstrous burn down your house spider. Um That will be the female. They have an interesting reproduction too. So you should just go Wikipedia them. They're pretty they're pretty great. Okay. And then you have sex influence inheritance. And this is when the sex of an individual influences the expression of a phenotype. So this means that the phenotype can exist in both populations, but it might be more severe in one than in one sex than the other. So usually these types of genes are dependent on hormones that are different in males and females. So an example of this is pattern baldness. Now we all know pattern baldness can affect males and females. But obviously um all those like tv hair replacement commercials suggests it affects males much more. Um And that's because these genes are sex influence meaning that the hormones produced by the male. Actually, you know, leave these phenotype to be much more severe in males. So baldness is much more common and much more severe. So those are two different terms. Now we're going to move on to an example but we're actually gonna do this in the next little short video. So with that let's now move on

Morgan's Eye Color Fruit Fly Cross

Okay, so now I want to give you an example that you're going to read about a lot in your book and you're going to see this in lecture and see this is lab. If you have a lab and you're just gonna have to know it. So I suggest you got to just memorize this cross because you will be tested on it, I guarantee. And this has to do with X. Linkage and eye colors and the fly Drosophila. So these are fruit flies and I have very clearly written out what these crosses are and what your professor is talking about in case they're not explained very well. So generally what these crosses are done is to look at X linkage. So this is an X linked trait meaning that the mutation is on the X chromosome and therefore will be inherited differently in males and females. So first we're dealing with the parental trait. And so this is going to be a red eyed female. Let me color these red eyes here. And a white eyed male and I have given you the genotype. And the reason I've given you this is so that you understand that this is that red is dominant and white is the movement. Now, remember how you write these? Is that the plus sign represents what wild type? Right? Remember wild type and the absence of a plus side equals. Mean, now we're dealing with this on the X chromosome. So in females you're going to see two alleles. So you'll see two Ws. And females draw a line here. But you always see one W And one Y. And males. And this is because the males only have one, they have a Y. Chromosome and an X. Whereas females have two X. Is. So when we look at our parental, what you can see here is that the red eyed female that were crossing has 212 wild type alleles. And the white I mail. We're crossing has one mutant and one Y. So to get the F1 generation, if we were to make a Punnett Square, I'm just gonna draw one out here. How would we do this using the parental, we take one allele from the mother or the female and one for the mail for each column. So we would get for the female and we would get the mutant allele and the wide chromosome for the male. So then if you cross these, you get, sorry, this is supposed to be W. W. Let me move out of the way. Okay, so that's what you get. If you do the punnett square. Now, what does this say? Well, first we have these half are female and these have our mail. Now, if I were to ask the question, how many are wild type and or how many present wild type red eyes and how many percent white? You would ask. Okay. Do any of them have pluses? Well, here's a plus, here's a plus, here's a plus and here's a plus. So all of them are this phenotype. So even though there's red or even though there's males and females, all of them have red eyes because all of them have a wild type allele. Now. Generally what happens here is you cross the F one. So yeah and their siblings, but it's flies. So it doesn't matter. So you take a red eyed female F one and made it with its brother, then you look at the F two. So if we were to do this punnett square, what would that look like? You see genetics is a lot of pundits squares. So we have female and male. and so our two alleles would be right. So if we were to do the planet square here, what do we get to get half female half male? And we know this because the presence of the why? And we get 3/4 that look like wild type because each one of these has a plus and this one doesn't it's sad, it's immuned. So we have 3/4 red eyes, males and females and 1/4 wide eyed males only males. Because remember here are the females, they both have a X chromosome that is has the normal Alil and this one only has one chromosome. So it only has half the chance of getting that normally. Well and so it did not and therefore is now sad and it's a mutant. So this is the first cross and this is the cross starting with the red eyed female and the white eyed male and understanding what the parental czar are super important because you can do what's called a reciprocal cross which is we're going to do that next and that is when you take the opposite sexes. So now we're dealing with a red eyed male and a white eyed female. This is a very different cross and it's going to get completely different results. So when you're asked about these crosses on your exam, you need to make sure you understand which crosses it. Talking about it. Talking about the red eyed or the wide eyed female or the wide eyed male because you're gonna get different results. So let's walk through this cross. So now this cross is very different. This is the parental still we're looking at the red eyed male and the white eyed female. So first thing you wanna do is do the punnett square. They can draw it right there we go. So here's our punnett square. So we're going to take the mail which is still going to be down here. Why? And the female which is mutant so it lacks no Plus plus equals wild type absence of plus equals mutant. So when we do these crosses, what we get is you still get females still get males but you'll notice that only the females end up red eyed, that all of the males that are produced end up white eyed. So here's the red eyed females and this is very different. You remember the other cross it was everything was red. This cross you get one half to one half then you can do the F. To the F. To generally is you can do this a bunch of different ways. Well actually no you either cross, you have to cross the red eyed female with a wide eyed male. Again. So we're doing this planet cross. Lots of planet crosses what you get. Actually let me make that bigger. So I don't have to be that horrible person who scrunches up her riding and then you can't read it. So again hopefully you're getting your squares down and you understand where I'm getting these from. These are again wanna leo from each, this is the mother here, this is the father here and you do this cross. Yeah. Mhm. Um So you get males, you get females and um what do you end up with, you end up with one half red eyed females or males right? Because you can get here's the red eye and then here is the white eyed and you have both males and females. And here are the different genotype for them. I get it can be confusing because when we dealt with two crosses here, the reciprocal als one where the parental male would have red eyes and one where the parental female have red eyes. That can be confusing and then the F. One and F two generations are different But if you understand these punnett squares every single time I did one, if you understand where these are coming from, which I wrote them here for you and you understand you know that these are males because they have the Y and these are females, they have two Ws. And you understand that the plus is going to be the wild type which in this case will be red and the absence of the plus will be immuned this case. White. Then you can pretty much answer any question that you may be given about this cross. So hopefully that's clear. I understand it could be a little confusing dealing with these crosses. But hopefully if you want to look back at this, just review it anytime you know your professors talking about it or you may have a quiz question about it. This is a good review because it deals with every has every generation. It has every genotype it has every phenotype. So hopefully that's clear with that. Let's not move on.

Which of the following sex chromosome pairs is caused from nondisjunction?

• A. XX
• B. XY
• C. ZZ
• D. XXY

Which of the following is an example of sex-limited inheritance?

• A. Pattern baldness in humans
• B. Male doves are white, while female doves are brown
• C. Klinefelter's disease
• D. Men tend to be taller than women