Sex Chromosome - Online Tutor, Practice Problems & Exam Prep

Sex determination varies across organisms, with two main types: homogametic (e.g., XX in females) and heterogametic (e.g., XY in males). Systems include XX/XY, XX/X0, and ZZ/ZW. Nondisjunction during meiosis can lead to disorders like Klinefelter syndrome (47,XXY) and Turner syndrome (45,X). In females, one X chromosome is inactivated, forming a Barr body, a process described by the Lyon hypothesis. This can result in mosaic phenotypes, as seen in calico cats, where different X alleles express distinct traits.

Sex Determination

So, first, let's talk about sex determination, which you may think, oh, that's so easy, XX, XY. I know. Female, male. But, actually, different organisms determine sex differently. Now, there are 2 types of sexes that we're going to talk about. The first is the homogametic sex, and this is a sex that only uses one type of chromosome. Now, this is referring to types of chromosomes, not numbers. The homogametic sex could have 30 X chromosomes and still be a homogametic sex because it has only X's. So, for instance, human females are an example of this. Then the second sex is called a heterogametic sex, and this is a sex that has two types of chromosomes. For us, this is X and Y. Now, remember, just like the homogametic sex, there could be 30 X chromosomes and 20 Y chromosomes, but it's still a heterogametic sex because there are only 2 types, no matter how many numbers there are. So, human females are the homogametic and human males are heterogametic. Now, sex determination systems are different in different organisms. Determination systems. So, one type is called the XXX0. And this is when the sexes are determined by having either 1 or 2 of the same chromosomes. So either they have 2 X's or they have one X. And that determines male and female. We have the one that we're most similar or familiar with. This is XXXY. And this is 1 or 2 different types of chromosomes. And this is obviously the one we're most familiar with because this is humans. XX is female, and XY is male. And then, you have this unusual one called, ZZW, which you probably never heard of before. And, the reason that it uses Z's is that the male and females kind of switch spots. So, males are actually the homogametic sex, meaning that the male is ZZ, and the female is the heterogametic sex, which means that it's ZW. And this is different than humans where the female is the homogametic and the male is the heterogametic. So this is given this special label. And I'll show you an image of what that looks like in a second. And then organisms can be classified as monoecious, and this is where an organism contains both male and female sexual organs. These are sometimes called hermaphrodites. And then, dioecious, and this is when an organism contains either male or female, but not both. So that is some classifications of sexual determination systems in different organisms. So here's an example of the XX, XO, ZW sort of systems here. So the ones we're most familiar with is human, XXY. And you can see here that if you take each gamete, here's female, here's male. You do Punnett squares, you get 2 females, 2 males. Then, another example is the XXX0, these are common in insects. And if you do the same thing with the two females, which are XX, and the males, which are X blank, it's just O or whatever you want to say. You get 2 females and 2 males. And then finally, you have, ZW and ZZ. Now, this is different because ZW is the female, you have your female gametes, they're different, and then the male gametes are the same. So you get 2 males here and here and 2 females here and here. So in case you just need a cheat sheet or you're a little confused, remember the females are either going to be XX, either in this system or in this system or ZW in this one. And the males will be XY, X0, or ZZ, depending on which system you are using.

With that, let's now turn the page.

Human Sex Chromosomes

Okay. So now let's focus a little bit more on the human's X chromosome. Humans follow the XX and XY pattern with males determined by the XY chromosomes. Now, these chromosomes are determined by the gametes combinations that you get, and remember, gametes are formed through meiosis. In normal meiosis, along with all the other types of autosomal or other non-sex chromosomes, they line up during meiosis and separate into daughter cells. But the XX and the XY aren't copies of each other. Right? But they still line up and separate independently into the haploid daughter cell. So each daughter cell gets 1 chromosome, either the X or the Y. Sometimes this doesn't happen properly. The separation of the chromosomes doesn't occur, and this means that one daughter cell will get both the X and Y, and one will get none. This is given a special term called nondisjunction. This is when the chromosomes don't separate properly during meiosis. When nondisjunction occurs, there are many disorders, medical disorders, or genetic disorders that occur. For example, one you're going to hear about is called Klinefelter, and it's written as 47, because there are now 47 chromosomes rather than 46 because there are 2 X’s and a Y. This is an extra sex chromosome. These typically present as male because they have male genitalia, but they are usually infertile; they fail to produce sperm and have other phenotypic symptoms as well. You can observe characteristics like long fingers and arms and sometimes the development of breasts. This is due to the extra X chromosome. I've written 47XXY, but actually, Klinefelter's can sometimes present as even more X's, like XXXXY. In these cases, they typically present with much more severe phenotypes, both physically and factors like infertility. Another significant condition is Turner Syndrome, which is noted as 45X, meaning there is one less chromosome, having only one X. These individuals typically have female genitalia, along with other cognitive and sexual impairments. They are usually infertile and have much reduced cognitive abilities. Sometimes in these unusual cases, individuals can be mosaics, with somatic cells displaying two different numbers of sex chromosomes. For example, some of your cells could be 45X, resembling Turner Syndrome, and some might be a normal 46XY for a male. These situations are rare, but they do occur, and you may hear about them. So let me back up and explain karyotype. A karyotype looks like this; it's a map of the chromosomes of an individual. You can see that the autosomes, which are all of these here labeled 1 through 22, have 2 pairs. The sex chromosomes, which are the X and the Y, have an extra one. So, when you are XXY, remember which disorder that is? Right, it is Klinefelter's Syndrome. Now, remember, there can be multiple X's here, so if there was another X chromosome, it would still be Klinefelter's, just more severe. If there was just 1 X, no Y, what would that be? Turner Syndrome. So, you may be presented with a karyotype and asked to identify the disorder, so make sure you understand how to read these and know the number of sex chromosomes found in these disorders. Human sexes, male or female, handle the chromosomes differently. The Y chromosome determines maleness, obviously, because all males have the Y. The reason for that is because the Y chromosome has this special region called the sex-determining region, and there is a protein called the testes determining factor or TBF protein. And this protein is what allows males to develop the way they do. Females, on the other hand, get 2 copies of the X chromosome, but you only need 1. When males have 1 X chromosome, they have a Y, but it's small, and it doesn't really do anything other than just determine maleness. It doesn't really have many genes on it. But the X chromosome is really important, and it has a ton of genes on it. And if you have 2 copies of it, you get double the dose. So, in females who get 2 copies, one actually has to be shut off. They do this by forming these things called Barr bodies. Barr bodies are inactivated X chromosomes, inactivated because the DNA and the protein that make up that chromosome, which you may have heard of as chromatin, which equals DNA plus protein, condense. It becomes very compact, and because it's so compact, it inactivates. It means that the genes on it cannot be transcribed or made into proteins. So, every female has 2 X chromosomes, but one of them is inactivated in every single cell. When does this inactivation occur? It occurs not at fertilization but later in development, according to the Lyon hypothesis. It occurs very early in development but not at the very beginning. Thus, there are multiple different somatic cells present when this happens. Say you have X1 and X2, and we'll just say half the cells inactivate one and half inactivate the other. Normally, you wouldn't notice a difference, but sometimes in some organisms, the alleles on the X chromosomes are different. So, if you inactivate one, you get a different phenotype than if you were to inactivate the other. This results in what's called a mosaic phenotype because the cells aren’t all presenting the same phenotype; some present one and some the other. An example of this is a calico cat. Calico cats have different coloration determined by which X chromosome gets inactivated early in development. So if X1 has the black allele and X2 has the orange allele, half the cells that inactivate X1 will show orange because black is now inactivated. The cells that inactivate X2 will show black because they inactivated the orange gene. This is why calico cats display different regions of color as they grow, with some regions presenting orange and others black. Interestingly, calico cats, formed based on which X chromosome gets inactivated, are all female because a male only has one X and one Y and therefore doesn’t inactivate one of them. This trait connected to the inactivation of the X chromosome is very cool. Oh, sorry, didn't mean to zoom in for you, but there you go, you got a better picture of the cat. I think it’s really interesting.

Which of the follow sex chromosomes can be describes as homogametic?

• A. XX/XY
• B. ZZ/ZW
• C. XX/XO

In the ZZ/ZW sex determination system, the male is…?

• A. Heterogametic
• B. Homogametic
• C. ZW
• D. XY

In humans, which region on the Y chromosome determines maleness?

• A. Sperm producing factor (SPF)
• B. Testis determining region (TDR)
• C. Sex Determining Region (SRY)
• D. Barr Body (BB)

During meiosis the XY sex chromosomes segregate independently. Which of the following represents the gametes chromosomes after meiosis?

• A. Two haploid cells that each contain X or Y
• B. Two diploid cells that both contain an XY
• C. A single diploid cell that contains XY
• D. Four haploid cells – two contain X and two contain Y

Here’s what students ask on this topic:

What are the different sex determination systems in organisms?

Sex determination systems vary across organisms. The most familiar system is the XX/XY system, where females are XX (homogametic) and males are XY (heterogametic). Another system is the XX/X0 system, common in insects, where females are XX and males are X0 (one X chromosome). The ZZ/ZW system is found in birds and some reptiles, where males are ZZ (homogametic) and females are ZW (heterogametic). These systems determine the sex of the organism based on the combination of sex chromosomes they inherit.

What is nondisjunction and how does it lead to disorders like Klinefelter and Turner syndrome?

Nondisjunction is the failure of chromosomes to separate properly during meiosis. This results in gametes with an abnormal number of chromosomes. When such gametes fuse with normal gametes, they can produce offspring with disorders. Klinefelter syndrome (47,XXY) occurs when an individual has an extra X chromosome, leading to male genitalia but often infertility and other physical traits. Turner syndrome (45,X) occurs when an individual has only one X chromosome, leading to female genitalia but often infertility and cognitive impairments.

What is the Lyon hypothesis and how does it relate to X chromosome inactivation?

The Lyon hypothesis states that in females, one of the two X chromosomes in each cell is randomly inactivated during early development. This inactivation forms a Barr body, which is a condensed, inactive X chromosome. This process ensures that females, like males, have one functional X chromosome per cell. The inactivation can lead to mosaic phenotypes, where different cells express different traits depending on which X chromosome is inactivated, as seen in calico cats.

How do calico cats demonstrate X chromosome inactivation?

Calico cats exhibit a mosaic phenotype due to X chromosome inactivation. The gene for fur color is located on the X chromosome. In early development, one of the two X chromosomes in each cell is randomly inactivated. If one X chromosome carries the allele for black fur and the other for orange fur, the inactivation of different X chromosomes in different cells results in patches of black and orange fur. This phenomenon occurs only in females, as males have only one X chromosome.

What are Barr bodies and how are they formed?

Barr bodies are inactivated X chromosomes found in female cells. They are formed through a process called X chromosome inactivation, which occurs early in development. The DNA and protein of one X chromosome condense tightly, preventing the genes on that chromosome from being transcribed into proteins. This ensures that females, who have two X chromosomes, do not produce double the amount of X-linked gene products compared to males, who have only one X chromosome.