Sex-Linked Genes - Online Tutor, Practice Problems & Exam Prep

Sex-linked genes are located on sex chromosomes, X and Y, influencing inheritance patterns. The Y chromosome, smaller and hemizygous, contains the SRY gene, determining maleness. The larger X chromosome has numerous genes with non-sexual functions, present in both sexes. Pseudoautosomal regions on X and Y facilitate proper segregation during meiosis. Nondisjunction can lead to disorders from improper chromosome separation. X-linkage affects males and females differently, while sex-limited and sex-influenced inheritance describe traits expressed variably between sexes, such as size in orb-weaver spiders and pattern baldness.

Sex-Linked Genes

Hi. In this video, we're going to be talking about sex-linked genes. Humans have two sex chromosomes, the X and the Y. These are super important. They have different functions, they act differently, and combinations of these determine whether or not we are female or male. The Y chromosome has certain characteristics. One is that it's actually a very small chromosome, and it 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 SRY gene, and this gene is what determines whether or not you're male, and it is the maleness 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 called hemizygous, and this is because there's only one Y chromosome. So if you think of all the other chromosomes, they come in homologous pairs, 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 hemizygous because there's only one of them in an organism.

Now the X chromosome is different than the Y because the X chromosome is 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 contains many 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, XX, and XY is male. Both sexes have an X chromosome, and so that's why the X has many more genes for nonsexual functions because everyone will get them, whereas the Y is pretty much just a sex-determining chromosome.

However, the X and Y, even though they're not the same chromosomes, do contain one or actually two regions that can work together. So these are called Pseudoautosomal Regions 1 and 2. These are copies, essentially. One's in the X and one's in the Y. The purpose of these regions is to help pair them together so that during meiosis, these chromosomes can separate into gametes properly. Normally, the autosomal pairs have 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, avoiding having too many or too few. The X and Y are two different chromosomes, so they don't have that, just inherently, as they are not exact copies of each other. So they have to have special regions called the pseudoautosomal regions 1 and 2 that are copies of each other that allow them to pair and separate so that you get the appropriate amount of Xs and the appropriate amount of Ys, instead of, you know, too many or too few. That's what those regions are for, and so they can act as a pair and segregate equally into sperm, or in the case of XX, into eggs.

Obviously, this process doesn't always go as planned, and there's a term called nondisjunction, and this occurs when chromosomes fail to separate properly. Now you can see this in homologous pairs, but this example relates to sex chromosomes. You can see that some organisms can end up with three Xs or two Xs and a Y, and then others can end up with just one X or one Y. Obviously, none of these are desired because you either want XX or XY. You don't want this, like, three-combination or just the one. We'll go over these, what these disorders are called at another time. 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 Xs would therefore be a female.

Sex linkage is a term used to describe when the genes located on a sex chromosome have certain inheritance patterns. 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, while males only have one X, and so they'll always get the mutant. There are different types of sex linkage. The one I just described is called X linkage, and there are mutants on the X, and it's going to cause them to inherit differently. There's Y linkage, this is actually fairly rare. The reason is because the Y has many fewer genes than the X. But there is Y linkage, and that is mutant alleles in the Y, so only males would get this.

And then you have a couple of other more nuanced terms, and these are sex-limited inheritance. This is when the expression of a phenotype, some kind of trait, is absolutely limited to one sex. An example of this is in certain animal populations, 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-weaver spiders, the females are huge. I mean, they're huge spiders, like, scary spiders that you, like, burn down your house if you see them. But the males are actually tiny, and you may actually miss them. And so that's an example of sex-limited because the males will always be tiny, and that is a phenotype that's absolutely limited to the male orb-weaver spiders. Whereas, the big huge, like, monstrous burn-down-your-house spider, that will be the female.

They have an interesting reproduction too, so you should just look them up. They're pretty great. Okay. And then, you have sex-influenced inheritance. This is when the sex of an individual influences the expression of a phenotype. This means that the phenotype can exist in both populations, but it might be more severe in one sex than the other. So usually, these types of genes are dependent on hormones that are different in males and females. An example of this is pattern baldness. Now, we all know pattern baldness can affect males and females, but obviously, all those TV hair replacement commercials suggest, it affects males much more, and that's because these genes are sex-influenced, meaning that the hormones produced by the male actually lead these phenotypes to be much more severe in males. So baldness is much more common and more severe.

So those are two different terms. Now we're going to move on to an example, but we're actually going to do this in the next 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. You're going to see this in lab, if you have a lab, and you're just going to have to know it. So I suggest you kind of just memorize this cross because you will be tested on it. I guarantee. And this has to do with x linkage and eye colors in 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. So let me color these red eyes here, and a white-eyed male. And I have given you the genotypes. And the reason I've given you this is to so that you understand that red is dominant, and white is the mutant. 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 sign equals mutant. Now we're dealing with this on the x chromosome. So in females, you're going to see 2 alleles. So you'll see 2 w's in females. Draw a line here, but you'll only see 1 w and one y in males. And this is because males only have 1, they have a y chromosome and an x, whereas females have 2 x's.

So when we look at our parentals, what you can see here is that the red-eyed female that we're crossing has 2 wild type alleles. And the white-eyed male we're crossing has 1 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 male for each column. So we would get for the female and we would get the mutant allele and the y 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 half are male. Now, if I were to ask a question, How many are wild type? And or how many percent wild type red eyes and how many percent white? You would ask, Okay, do any of them have pluses? Well, here is a plus, here is a plus, here is a plus, and here is a plus. So all of them are this phenotype. So even though there's red or even though there are 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 f1. So, yeah, then they're siblings, but it flies so it doesn't matter.

So you take a red-eyed female f1 and mate it with its brother. Then you look at the f2. So if we were to do this Punnett square, what would that look like? You see genetics is a lot of Punnett Squares. So we have female and male. And so our 2 alleles would be Right? So if we were to do the Punnett Square here, What do we get? We get half female, half male, and we know this because of the presence of the y, and we get, 3 fourths that look like wild type because each one of these has a plus and this one doesn't. It's sad. It's mutant. So we have 3 fourths red eyes, males and females. And 1 fourth wide-eyed males. Only males. Because, remember, here are the females. They both have an x chromosome that has the normal allele. And this one only has 1 chromosome, so it only has half the chance of getting that normal allele, and so it did not. And therefore, is now sad and it's 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 parentals are is 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, you know, which cross is it talking about. Is it talking about the red-eyed or the white-eyed female or the white-eyed male? Because you're going to 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 want to do is do the Punnett Square. If I could draw it right. There we go. So here's our Punnett Square. So, we're going to take the male, which is still going to be down here, y, 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 wide-eyed. So here are the red-eyed females. And this is very different. You remember the other cross, it was everything was red. This cross, you get onetwo to onetwo.

Then, you can do the f2. And the f2 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 the white-eyed male. Again, so we're doing this Punnett cross, lots of Punnett crosses. What you get actually, let me make that bigger, so I don't have to be that horrible person who scrunches up her writing, and then you can't read it. So, again, hopefully you are getting your kind of squares down and you understand where I am getting these from. These are again, one allele from each. This is the mother, here. This is the father, here. And, you do this cross. So you get males, You get females. And, 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. Here are the different genotypes for them. Again, it can be confusing because, one, we dealt with 2 crosses here, the reciprocals. One where the parental male would have red eyes, and one where the parental female would have red eyes. That can be confusing. And then the f1 and f2 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 because they have 2 w's, 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 mutant, in 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 can 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 professor's talking about it or you may have a quiz question about it, this is a good review because it deals with every generation. It has every genotype. It has every phenotype. So, hopefully, that's clear. With that, let's now 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

Here’s what students ask on this topic:

What are sex-linked genes and how do they influence inheritance patterns?

Sex-linked genes are located on the sex chromosomes, X and Y, and they influence inheritance patterns differently in males and females. The Y chromosome is smaller and contains fewer genes, including the SRY gene, which determines maleness. The X chromosome is larger and contains numerous genes with both sexual and non-sexual functions. Because males have one X and one Y chromosome (XY), and females have two X chromosomes (XX), mutations on the X chromosome can affect males and females differently. For example, a male with a mutation on his single X chromosome will express the trait, while a female with one mutated X chromosome may not if her other X chromosome is normal. This difference leads to unique inheritance patterns for X-linked traits.

What is the SRY gene and what role does it play in sex determination?

The SRY gene, located on the Y chromosome, is crucial for sex determination in humans. SRY stands for Sex-determining Region Y, and it is often referred to as the 'maleness factor.' This gene triggers the development of male characteristics by initiating the formation of testes, which subsequently produce male hormones like testosterone. In the absence of the SRY gene, as in individuals with two X chromosomes (XX), the default development pathway leads to female characteristics. Therefore, the presence or absence of the SRY gene determines whether an individual will develop male or female sexual characteristics.

What are pseudoautosomal regions and why are they important?

Pseudoautosomal regions (PARs) are specific areas on the X and Y chromosomes that are identical and allow these chromosomes to pair and segregate properly during meiosis. There are two pseudoautosomal regions, PAR1 and PAR2. These regions are crucial because they ensure that the X and Y chromosomes can align and separate correctly into gametes, preventing disorders caused by improper chromosome segregation, such as Turner syndrome (one X chromosome) or Klinefelter syndrome (XXY). Without these regions, the unique pairing and segregation of the sex chromosomes would be compromised, leading to aneuploidy and associated genetic disorders.

What is nondisjunction and how does it affect sex chromosomes?

Nondisjunction is the failure of chromosomes to separate properly during cell division, specifically meiosis. When nondisjunction occurs with sex chromosomes, it can result in gametes with an abnormal number of sex chromosomes. For example, a gamete might end up with two X chromosomes or no sex chromosome at all. If such a gamete is fertilized, it can lead to conditions like Turner syndrome (45, X), where an individual has only one X chromosome, or Klinefelter syndrome (47, XXY), where an individual has an extra X chromosome. These conditions can cause various developmental and physical abnormalities.

What is the difference between sex-limited and sex-influenced inheritance?

Sex-limited inheritance refers to traits that are expressed in only one sex, even though the genes for the trait are present in both sexes. An example is the size difference in orb-weaver spiders, where females are significantly larger than males. Sex-influenced inheritance, on the other hand, refers to traits that are expressed in both sexes but are more pronounced in one sex due to hormonal differences. An example is pattern baldness, which can occur in both males and females but is more common and severe in males due to the influence of male hormones. These differences highlight how sex can affect the expression of certain genetic traits.