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

Sex-linked inheritance involves genes located on sex chromosomes, primarily the X chromosome, which is larger and contains more genes than the Y chromosome. Females have two X chromosomes, allowing for homozygous or heterozygous conditions, while males have one X and one Y chromosome, expressing whatever allele is present on their single X. X-linked recessive disorders, like hemophilia, affect males more frequently since they require only one recessive allele to express the disorder. Pedigrees for these disorders typically show a higher prevalence in males, with specific inheritance patterns for affected females.

Sex-Linked Inheritance

In this video, we're going to begin our lesson on sex-linked inheritance. Recall from our previous lesson videos that the sex chromosomes include the X or the Y chromosome. The sex chromosomes are going to be the ones that actually determine the sex of the organism, whether the organism develops male reproductive systems or female reproductive systems. Females are typically going to have 2 X chromosomes, whereas males, on the other hand, are going to typically have 1 X chromosome and one Y chromosome. Sex-linked genes are defined as genes that are found on either sex chromosome. So genes found on the X chromosome are X-linked genes and genes found on the Y chromosome are Y-linked genes. It's also important to note that the X chromosome is significantly larger than the Y chromosome, and we can tell just by comparing the number of genes on each chromosome. The X chromosome actually contains somewhere around 1100 X-linked genes, which is a lot in comparison to the Y chromosome which only contains about 100 Y-linked genes. Just by comparing the number of genes you can see that the Y chromosome must be significantly smaller since it has so many fewer genes than the X chromosome, which is going to be significantly larger.

If we take a look at our image down below, over here on the left-hand side, notice that we're focusing on sex-linked genes. You can see that here in red we are representing the X chromosome which is significantly larger than the little Y chromosome that we have right next to it. The black band that you see right here represents an X-linked gene because it represents a gene found on the X chromosome. This band that you see over here on the Y chromosome represents a Y-linked gene. Moving forward in our course we're mainly going to be focusing on the X-linked genes and not so much on Y-linked genes. Again, females tend to have 2 X chromosomes as you can see here, and males tend to have only 1 X chromosome and one Y chromosome. It's the presence of the Y chromosome that's going to contain genes that allow for the male reproductive systems to develop.

What's very important to note here in this video is that with each fertilization event, there's actually a 50% chance of having a female and a 50% chance of having a male. If we take a look at this Punnett square that we have over here on the right-hand side, what you'll notice is that we're crossing a mother with a father. Again, females tend to have 2 X chromosomes whereas males tend to have 1 X chromosome and one Y chromosome. When meiosis occurs and these gametes are formed and we complete the punnet square, what you'll notice is that in the offspring every time, there's a 50% chance of having a female and a 50% chance of having a male. This is exactly what we're indicating here, a 50% chance of having a female with 2 X chromosomes right here and a 50 percent chance of having a male with 1 X and 1 Y chromosome as we see in these two possibilities. This here concludes our introduction to sex-linked inheritance, but as we move forward in our course we're going to continue to talk more and more about these X-linked genes and sex-linked inheritance. So I'll see you all in our next video.

X-Linked Inheritance

In this video, we're going to talk about X-linked inheritance. It's important to note that because females have 2 X chromosomes, they have 2 alleles for each X-linked gene, where the females inherit one allele from each of their parents. Females can either be homozygous dominant, homozygous recessive, or heterozygous for the X-linked genes. However, this is not the case for males, because males only have one X chromosome and a Y chromosome, therefore, they only have one allele for each X-linked gene instead of having 2 alleles like females do. Males inherit one of their alleles from their mother and none of the alleles from their father because they inherit a Y chromosome from the father. So, males cannot be homozygous dominant, recessive, or heterozygous. Instead, males express whatever X-linked allele is on their single X chromosome inherited from their mother.

In our example below, this experiment tracking eye color in fruit flies first revealed the X-linked inheritance pattern. There are two possible eye colors in this experiment: red eyes and white eyes. The gene for eye color is on the X chromosome, making it an X-linked gene with two alleles. The dominant allele is \(X_R\) which results in red eyes, and the recessive allele is \(X_r\) which leads to white eyes, with \(r\) being recessive to \(R\) which is dominant.

Let’s look at the different Punnett Squares we have. In the first cross, we have a homozygous red-eyed female (\(X_RX_R\)) crossed with a white-eyed male (\(X_r\)). When completing the Punnett Squares, keep in mind that males will pass on their Y chromosome in 50% of the cases. What results is that \(X_R\) is brought down in the Punnett Square, while the \(X_r\) and the Y chromosome are passed on. Therefore, 50% of the offspring will be males and 50% females, with none of the females having white eyes and none of the males too in this specific cross.

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In another cross, we have a homozygous white-eyed female (\(X_rX_r\)) with a red-eyed male (\(X_R\)). Here, all females inherit \(X_R\) from the father and \(X_r\) from the mother, leading to a heterozygous red-eyed outcome. However, for males, since they inherit the mother’s \(X_r\) and the father’s Y, 100% of the males will have white eyes.

In a different scenario where a heterozygous red-eyed female (\(X_RX_r\)) is crossed with a red-eyed male (\(X_R\)), you’ll find that 50% of the males have white eyes but no females do. In the last situation, crossing a heterozygous red-eyed female with a white-eyed male (\(X_r\)), results in 50% of both females and males having white eyes.

This experiment illustrates the different inheritance patterns for males and females, which was a key observation by scientists studying these traits. We'll discuss further how X-linked disorders typically affect males more than females as we progress in our course. These patterns are foundational for understanding genetic inheritance and will help us apply these concepts moving forward. See you all in our next video.

Wild type fruit flies have red eyes. A white-eyed female fly is crossed with a red-eyed male fly. All of the females from the cross are red-eyed and all of the males, white-eyed. What type of inheritance pattern is this?

• A. Autosomal recessive.
• B. Autosomal dominant.
• C. Sex-linked on X chromosome.
• D. Incomplete dominance.
• E. Sex-linked on Y chromosome

When Thomas Hunt Morgan crossed red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

• A. The gene involved is located on the Y chromosome.
• B. The gene involved is located on the X chromosome.
• C. The gene involved is located on an autosome, but only in males.
• D. Other male-specific factors influence eye color in flies.

X-Linked Recessive Disorder: Hemophilia Inheritance

In this video, we're going to talk about X-linked recessive disorders as we discuss a specific example in hemophilia inheritance. Hemophilia is a disorder that's characterized by abnormal blood clotting, and it is an X-linked recessive disorder found in humans. By X-linked recessive disorder, the X-linked part means that the disorder is associated with the X chromosome and a gene on the X chromosome. By recessive, it means that it's only going to be expressed if the individual only has the recessive allele or the recessive alleles on the X chromosome or X chromosomes. Females must be homozygous recessive or have 2 recessive alleles to be affected by an X-linked recessive disorder. However, males require only one recessive allele to be affected because they have only one X chromosome and need only receive 1 recessive allele to be affected. This makes males much more likely to be affected by X-linked recessive disorders, a characteristic of these disorders, as they tend to affect males more than females.

If we examine our image down below, notice that we're showing you a Punnett square crossing a heterozygous mother unaffected by hemophilia with an unaffected father. The heterozygous mother will have an X^H and an X_h. It's the X_h that is associated with the disorder. A mother with an X^H, a female with an X^H, is saved from having the disorder. Females need to have 2 X_h alleles to be affected, whereas males need only one X_h to be affected. Completing this Punnett square, we can bring down the X^H to these areas. We will have the X and the X here, each with uppercase H's. Additionally, we can do the same with the X_h, bringing that down so the males, positioned here and here, will each carry a Y chromosome. What you will notice, observing the females in the top half, is that they are unaffected because they have at least one capital H, one X^H. Regarding the males in the lower half, 50% of them will be unaffected, but 50% will be affected, or that's the likelihood.

Hemophilia, characterized by the inability to form blood clots and abnormal blood clotting, concludes our introduction to X-linked recessive disorders, specifically hemophilia inheritance. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Women with X-linked disorders always pass the genes for the disorder on to ______. While men with X-linked 

disorders always pass the genes for the disorder on to _______.

• A. only their daughters; only their daughters.
• B. both their daughters and sons; only their sons.
• C. both their daughters and sons; only their daughters.
• D. both their daughters and sons; both their daughters and sons.

X-Linked Recessive Pedigrees

In this video, we're going to talk about the X linked recessive pedigrees. And so X linked recessive pedigrees are of course going to be pedigrees that depict an X linked recessive disorder. Typically, these X linked recessive pedigrees show that more males are going to be affected than females. Also, females of an X linked recessive pedigree can only be affected if the father is affected and the mother is either affected or at least a carrier or is at least heterozygous for the disorder. All sons of an affected female will be affected when it comes to X linked recessive disorders.

If we take a look at our example below, we can see a pedigree of an X linked recessive disorder. We can label this as an X linked recessive disorder, specifically looking at red-green color blindness. Red-green color blindness is an X linked recessive disorder. Having an X_B means that the individual will have normal vision and will not be color blind with this allele. However, it's the X_b that leads to having this particular color blindness. Because it is an X linked recessive disorder, females are going to need to have two X_bs in order to be affected, whereas males only need one X_b to be affected. The affected individuals are the ones that have the filled-in black boxes. You'll notice that all of the individuals with a filled-in black box have X chromosomes only with the recessive allele. Notice that there is a large tendency for males to be affected more than females, and the vast majority of the people affected in this case are males. There's only one female that is affected in this scenario, and that is a pattern that will help you identify X linked recessive disorders much more quickly, just by looking to see if more males are significantly affected than females.

Notice again that a female can only be affected if the father is affected and the mother is at least a carrier. When we look at the affected female, we see that the father is affected, and the mother is a carrier, meaning that she is heterozygous. That's the only way a female can be affected. All the sons of an affected female are going to be affected. Notice that the only son of this couple is affected because the mother is affected.

This concludes our introduction to X linked recessive pedigrees, and the biggest takeaway should be that more males are going to be affected than females, which is a big clue that can help you identify X linked recessive pedigrees more quickly. This here concludes this lesson, and I'll see you all in our next video.

Sex-Linked Inheritance Example 1

So here we have an example problem that's asking what is the inheritance pattern of the following pedigree over here on the right hand side. And we've got these 5 potential answer options down below. Now right off the bat, we know that we can probably eliminate answer option e since we didn't really talk a lot about Y-linked disorders in our previous lesson videos. Now, one of the first things that you want to try to do to eliminate any of the X-linked disorders is to look at the distribution of the disorder across males and females. And so when you count up all of the individuals that are affected and not affected, what you'll realize is that there are 5 females that are affected, and there are 7 males that are affected. Again, the females are the shaded circles, the affected females are the shaded circles, and the affected males are the shaded squares. And so there are 5 shaded circles and 7 shaded squares, and that's pretty close to each other. So because there's not a massive distribution here between males and females, then we might eliminate the X-linked patterns of inheritance. So that limits us to either an autosomal dominant or an autosomal recessive pattern of inheritance. And recall that one of the key features of autosomal dominant is that it is found in pretty much every generation. It tends to be found in every generation. And so when you look across generation 1, you can see that it is certainly found here. You look across generation 2 and at first glance it does seem to be found in this generation. But what you'll realize is that individual number 7 here is not actually linked vertically to these parents, which means that this individual 7 is actually coming from another family. And so really, what we can see is that there has been a skip in the generation for the individuals that are connected vertically here. And so a skip in the generation is a symbol of or a sign of autosomal recessive, and it turns out that this is actually the case for this problem. And so what you can see is that the disorder is in generation 1, it skips generation 2, and then it's found in generation 3. And down below here, you can kinda see over here that it skips this entire side over here in this generation. And the only reason that it may not have skipped over here is that there was an individual that was also affected, that came into play. But again, this is how we can also determine that it is autosomal recessive. Now another way to check for this is we know that autosomal recessive means that the individuals must be homozygous recessive in order to have the disorder. And so you could just put in some letters in here to figure this out and of course, the individuals that are not affected must have at least one dominant allele. So you could do that throughout and then just see if this holds consistency throughout the entire thing. And you could perform pedigrees to check to ensure that it is actually possible for this pattern of inheritance to exist throughout this pedigree. But again, the easiest way is to just check for these clues such as the skipping of a generation or the presence of the disorder in every generation. And so this here concludes this example, and we'll be able to get some more practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

The following pedigree is for the X-linked-recessive trait for color blindness. Using XN for the normal allele and Xn for the color blindness allele, fill in the top half of the boxes/circles with the genotype. Also, fill in the bottom half of the boxes/circles with the phenotype (Normal vision or color blind). If it is impossible to know for certain a specific allele in the genotype, then place a '?' to represent the allele that is in question.

Okay, everyone. We have another big pedigree question. The following pedigree is for the x-linked recessive trait for color blindness. Using XN for the normal allele and Xn for the color blindness allele, fill in the top half of the boxes or circles with the genotype. Also, fill in the bottom half of the boxes or circles with the phenotype, normal vision or color blind. If it is impossible to know for certain the specific allele in the genotype, then place a question mark to represent the allele that is in question. They tell us that XN is normal, and they say that Xn is color blind, which I'm going to just write because the boxes aren't very big for me, so there's not a lot of room to write. Okay. So now I'm not going to lie to you. These are going to be more challenging than the ones we've done before. Because they're x-linked, there are different probabilities for males and females. And usually with these types of pedigrees, you can't just start at the top and work your way down. You're going to have to find an individual that you know their genotype and work backward from there. So where I'm going to start is not at the top. I'm going to start here. This is a female, and she is color blind, which means that she not only has one of her X's with the color blind allele, but both of them. So her genotype has to be XnXn for her to be colorblind. Okay. So now let's work back from that. We can see that her brother right here is normal. So he has to have XNY. Okay? So he is normal. We do not know about their other brother. So let's just ignore him for right now. We don't know about him. Okay. So we know these two siblings. So that means that the color blind daughter had to get a color blind allele from her mother and her father, which means that her father right here has a color blind allele, which means he is color blind. Okay? CB for color blind. Okay. So now we also know that the mother's phenotype is normal vision. And she must have given her normal allele to her son. So she has one normal allele, but because she had a color blind daughter, that means she must have donated her color blind allele, so she is a heterozygous individual. Okay. So now, let's go back even farther. So let's do the mother's family. Okay? So we're going to do this mother's family. So she is normal, but she's a heterozygous individual. We're told that her brother is normal, so he must be XNY. And we're told that her sister is normal, and we were given her genotype. Whoops. Okay. So he is normal. Her sister's normal. Her father is normal. So he has XNY. So what is her mother? Okay. So first off, one of the daughters here and the son have a normal phenotype. So remember the son gets his X chromosome from his mother, and it was normal. So she has to have at least one normal allele. But because she has a daughter who is a heterozygous individual for that recessive colorblind allele, that means that the mother must also be heterozygous. So that means that this daughter, let me point to her, this daughter, right here, got the normal allele from the dad and the recessive colorblind allele from the mom. Okay? So that is that family. So he is color blind. His brother has normal vision. His sister has normal vision. Okay. His father has normal vision, and his mother has normal vision, but he has a colorblind allele right there. Remember that males only get their colorblind alleles from their mothers, so she must have at least one colorblind allele. But because she has normal vision, she also has a dominant normal vision allele. So she is a heterozygote. These women are commonly called carriers because they carry the gene, but they don't have the phenotype. So she is a carrier. She has a colorblind son, a normal son. What about the daughter? Her vision is normal. Now, we're told that means that she had to get this normal allele from her dad, and we don't know if she got the normal allele from her mom, or the colorblind allele from her mom. Either way, she would have normal vision. So we're going to put a question mark here. And because she doesn't have any children, we can't verify for sure which allele she has. Okay? So there's a question mark there because we don't know what her other allele is. So we finished with that family. Now, let's move down the tree. Okay. So let's look at another family. We know that this mother here is color blind. Right? And we know that her husband is normal. So his genotype has to be XNY. Okay. So now we're also told that this daughter is normal, which means she has to have at least one dominant allele. And she got that one from her father. Because if you notice, the mother, right here, can only donate the recessive colorblind allele, so the daughter must have a recessive allele, so that normal daughter must be heterozygous. Okay? Again, this whoops. Let me get my highlight. This normal daughter here, she has normal vision, so she got her dominant normal allele from her father. And because her mother can only give her the recessive alleles, she has one recessive allele, so she is a carrier. What about their son? What about this guy right here? Well, remember that males only get their X chromosome from their mothers. Alleles to give, so her son is colorblind. So we're going to write CB right there. Okay? Oh, I also forgot up here, I'm sorry, I forgot this. Remember that this woman, because she's heterozygous, has to be normal. Let me write I'm just going to write norm. She has to be normal because she's heterozygous. Okay? Okay. So we've finished almost all of the families. We have one family left. So let's get rid of these highlights. Okay. So now we don't know anything about these two parents here. But the thing that we do know is we know that their daughter is color blind, which means she only has one genotype, and that means she is homozygous recessive. So that means that the father gave his only X chromosome to her, which means his X chromosome must be recessive, which means he must be color blind. Okay? Alright. Now, the other thing we know is right here is this son is normal. What is his genotype? It has to be XNY. His X allele is dominant. He has the normal allele, and he has a Y. So what does this tell us about this mother? Well, that tells us because her son has a normal genotype, that she gave him that normal genotype. But because she has a daughter who is color blind, she had to give a recessive allele to her daughter. So she has normal vision, but she is a heterozygous carrier. Okay, everyone. We have filled out all of the genotypes and all of the phenotypes for this family. And this family does, in fact, carry color blindness. And like I said, these sex-linked ones are a little bit more complicated. Practice these, and I know you will be very good at them. Okay, everyone. Let's move on to our next video.

Here’s what students ask on this topic:

What is sex-linked inheritance and how does it differ between males and females?

Sex-linked inheritance involves genes located on the sex chromosomes, primarily the X chromosome. Females have two X chromosomes, allowing them to be homozygous or heterozygous for X-linked genes. This means they can carry two alleles for each gene, one from each parent. Males, on the other hand, have one X and one Y chromosome. They only have one allele for each X-linked gene, inherited from their mother, and express whatever allele is present on their single X chromosome. This difference makes males more susceptible to X-linked recessive disorders, as they only need one recessive allele to express the disorder, whereas females would need two.

Why are X-linked recessive disorders more common in males than females?

X-linked recessive disorders are more common in males because males have only one X chromosome. For a male to express an X-linked recessive disorder, he needs only one copy of the recessive allele on his single X chromosome. In contrast, females have two X chromosomes and would need two copies of the recessive allele to express the disorder. This makes males more likely to be affected by X-linked recessive disorders, such as hemophilia and red-green color blindness, as they have a higher probability of inheriting the single necessary recessive allele.

How does a Punnett square illustrate the inheritance of X-linked traits?

A Punnett square can illustrate the inheritance of X-linked traits by showing the possible combinations of sex chromosomes and alleles from each parent. For example, if a mother is a carrier for an X-linked recessive disorder (X^HX^h) and the father is unaffected (X^HY), the Punnett square will show the potential genotypes of their offspring. The possible combinations are X^HX^H (unaffected female), X^HX^h (carrier female), X^HY (unaffected male), and X^hY (affected male). This helps visualize the 50% chance of having a male or female and the likelihood of inheriting the disorder.

What are some examples of X-linked recessive disorders?

Examples of X-linked recessive disorders include hemophilia, red-green color blindness, and Duchenne muscular dystrophy. Hemophilia is characterized by abnormal blood clotting due to a deficiency in clotting factors. Red-green color blindness affects the ability to distinguish between red and green colors. Duchenne muscular dystrophy is a severe form of muscular dystrophy that leads to muscle degeneration and weakness. These disorders are more common in males because they only need one copy of the recessive allele on their single X chromosome to express the disorder.

How can pedigrees be used to track X-linked recessive disorders?

Pedigrees can be used to track X-linked recessive disorders by illustrating the inheritance patterns across generations. In these pedigrees, affected individuals are typically shaded. More males than females are usually affected due to the nature of X-linked recessive inheritance. Females can only be affected if their father is affected and their mother is at least a carrier. All sons of an affected female will be affected. By analyzing these patterns, one can determine the likelihood of individuals being carriers or affected by the disorder.