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Sex-linked traits

Sex-linked traits are characteristics that are determined by genes located on the sex chromosomes. Inheritance patterns for these traits can differ for males and females, since the X and Y chromosomes differ. One example of a sex-linked trait is color blindness, which is primarily seen in males. X-inactivation can also influence the expression of sex-linked traits. Created by Sal Khan.

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  • blobby green style avatar for user Kanika
    So if a woman has hemophilia and has a son, then he would have to have hemophilia too, right?
    (99 votes)
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    • male robot hal style avatar for user Rudain75
      That's correct! Ok so Khan Academy doesn't like a short answer so I will go a little in depth.

      Basically, the mother donates one of her X chromosomes. Since she is a hemophiliac both of her X chromosomes have the allele for it. The husband doesn't really matter here. He could be free of hemophilia but sense he donates the Y chromosome for a boy it doesn't really matter.
      (127 votes)
  • male robot hal style avatar for user David Wu
    What is hemophilia?
    (16 votes)
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    • mr pants teal style avatar for user erikgaas
      Hemophilia is a disease that makes it difficult for your blood to clot. So if someone with hemophilia were to get cut, this would probably make them bleed for a long time before it stopped. That fact that it is sex-linked though makes it particularly interesting. Since women have two x chromosomes, they are less likely to suffer from homophilia because they have an extra x chromosome which will be able to make the clotting protein correctly even if the other can't. Men do not have that type of insurance. If their x chromosome is defective in that way then they will have it.
      (86 votes)
  • purple pi purple style avatar for user Sammy Cortes
    are the x and y chromosomes part of the 22 pairs of chromosomes (the autosomal) or would that be a 23rd pair???
    (13 votes)
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  • blobby green style avatar for user Le Scott
    can someone have 2 y chromosomes
    (19 votes)
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    • leaf green style avatar for user Alan
      Technically yes, as there is a condition known as XYY syndrome where an error in sperm cell formation will leave a sperm cell with two Y chromosomes. However, no one can only have two Y's, as the mother's egg will always provide an X.
      (41 votes)
  • piceratops tree style avatar for user lilypippili
    ive heard about people with too many or too few sex chromosomes, but what happens if a person doesn't have any sex chromosomes at all?
    (18 votes)
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    • female robot ada style avatar for user MBerkley
      if a person didn't have any sex chromosomes, then they wouldn't be able to survive. to live, a person must have at least one X chromosome because it has the most information on it. there are people who have maybe an XXY sex chromosome, because either the father's sperm cell or the mother's egg cell had a problem during meiosis. but to sum it up, if you had no sex chromosomes, the blastocyst would never develop and the mother would have never know she was pregnant
      (21 votes)
  • aqualine ultimate style avatar for user Mayank Rishu
    How many genes does a chromosome have? Is tht same in all?
    (10 votes)
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    • male robot hal style avatar for user Rudain75
      No, each chromosome has a different number of genes. One thing to remember is that chromosomes are numbered according to their size. With the first chromosome being the largest and the smallest one is 22. There is a small exception here with the X and Y chromosomes. As a result, the larger chromosomes have more information and therefor probably more genes.
      (13 votes)
  • orange juice squid orange style avatar for user LaFontaine
    Are there any other diseases like hemophilia which affects only x or y chromosome?
    (6 votes)
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  • blobby green style avatar for user michael weitzhandler
    Since X chromosome inactivation occurs in females, why is it that females who are carriers for x-linked diseases don't express the disease? I know that the inactivation occurs early in development, but if 50% of the cells express the allele that can pass on the disease, shouldn't the expression of these alleles result in expression of the disease?
    (5 votes)
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    • male robot hal style avatar for user Satwik Pasani
      Excellent question. I had this question from quite a long time. The answer is that heterozygous carrier females do express some symptoms of the disease, but the few cells where the functional allele is not inactivated produce enough protein to prevent a full-blown syndrome, and in many cases, keeps the disease un-noticeable. In haemophilic females, there is some deficiency of the clotting factors and this deficiency varies greatly in different females. Many a times, when the disease-gene is at a specific locus. Such locations may escape lyonization (X-inactivation) and might still transcribe proteins even if they are on the x-chromosome chosen for inactivation. This escape is quite well-documented in certain cases. In others, such as colour blindness, the few cells expressing the normal phenotype compensate for the others expressing diseased phenotype (in this case, the phenotype is colour sensitivity of cones in retina). If none of these work, heterozygotes for x-linked recessive (or dominant) will have an intermediate phenotype, somewhere between normal and diseased. see http://www.nature.com/scitable/my-discussion/2181 if you have a scitable account. But, I have summarized all the essentials.
      (8 votes)
  • leaf green style avatar for user Michael Parsons
    Women have two x chromosomes, each with about 1,500 genes, while men only have a x chromosome and a y chromosome, with only about 78 genes. Could this be a factor in why women live longer than men?
    (6 votes)
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    • leaf green style avatar for user magriekitten
      I do not believe that women live longer due to having 2 X chromosomes. A woman's somatic cells turn off one of the X chromosomes on random. This is called a Barr body. The only place the 2 X chromosomes come into play is in the meiosis of germ cells. Therefore, while women get more genes through 2 X chromosomes, it is not necessary for the women to "use" all the genes from the sex chromosomes.
      (3 votes)
  • orange juice squid orange style avatar for user sruti.anandi
    My teacher says it is impossible for a woman to have hemophilia as a man having hemophilia cannot survive for long. And as a result the female cannot marry the man having hemophilia and cannot produce female offspring having hemophilia. Is this explanation correct?
    (2 votes)
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    • orange juice squid orange style avatar for user CarlBiologist
      No. Plenty of men with the disease survive long enough to have children, especially with modern medicine. Usually only men get the disease because it is caused by an allele for a gene located on the X chromosome. Men only have one X chromosome whereas women have two. The allele is rare enough in the population that a woman is unlikely to have two copies of the allele and thus women are usually unaffected.
      (12 votes)

Video transcript

By this point in the biology playlist, you're probably wondering a very natural question, how is gender determined in an organism? And it's not an obvious answer, because throughout the animal kingdom, it's actually determined in different ways. In some creatures, especially some types of reptiles, it's environmental. Not all reptiles, but certain cases of it. It could be maybe the temperature in which the embryo develops will dictate whether it turns into a male or female or other environmental factors. And in other types of animals, especially mammals, of which we are one example, it's a genetic basis. And so your next question is, hey, Sal, so-- let me write this down, in mammals it's genetic-- so, OK, maybe they're different alleles, a male or a female allele. But then you're like, hey, but there's so many different characteristics that differentiate a man from a woman. Maybe it would have to be a whole set of genes that have to work together. And to some degree, your second answer would be more correct. It's even more than just a set of genes. It's actually whole chromosomes determine it. So let me draw a nucleus. That's going to be my nucleus. And this is going to be the nucleus for a man. So 22 of the pairs of chromosomes are just regular non-sex-determining chromosomes. So I could just do, that's one of the homologous, 2, 4, 6, 8, 10, 12, 14. I can just keep going. And eventually you have 22 pairs. So these 22 pairs right there, they're called autosomal. And those are just our standard pairs of chromosomes that code for different things. Each of these right here is a homologous pair, homologous, which we learned before you get one from each of your parents. They don't necessarily code for the same thing, for the same versions of the genes, but they code for the same genes. If eye color is on this gene, it's also on that gene, on the other gene of the homologous pair. Although you might have different versions of eye color on either one and that determines what you display. But these are just kind of the standard genes that have nothing to do with our gender. And then you have these two other special chromosomes. I'll do this one. It'll be a long brown one, and then I'll do a short blue one. And the first thing you'll notice is that they don't look homologous. How could they code for the same thing when the blue one is short and the brown one's long? And that's true. They aren't homologous. And these we'll call our sex-determining chromosomes. And the long one right here, it's been the convention to call that the x chromosome. Let me scroll down a little bit. And the blue one right there, we refer to that as the y chromosome. And to figure out whether something is a male or a female, it's a pretty simple system. If you've got a y chromosome, you are a male. So let me write that down. So this nucleus that I drew just here-- obviously you could have the whole broader cell all around here-- this is the nucleus for a man. So if you have an x chromosome-- and we'll talk about in a second why you can only get that from your mom-- an x chromosome from your mom and a y chromosome from your dad, you will be a male. If you get an x chromosome from your mom and an x chromosome from your dad, you're going to be a female. And so we could actually even draw a Punnett square. This is almost a trivially easy Punnett square, but it kind of shows what all of the different possibilities are. So let's say this is your mom's genotype for her sex-determining chromosome. She's got two x's. That's what makes her your mom and not your dad. And then your dad has an x and a y-- I should do it in capital-- and has a Y chromosome. And we can do a Punnett square. What are all the different combinations of offspring? Well, your mom could give this X chromosome, in conjunction with this X chromosome from your dad. This would produce a female. Your mom could give this other X chromosome with that X chromosome. That would be a female as well. Well, your mom's always going to be donating an X chromosome. And then your dad is going to donate either the X or the Y. So in this case, it'll be the Y chromosome. So these would be female, and those would be male. And it works out nicely that half are female and half are male. But a very interesting and somewhat ironic fact might pop out at you when you see this. Who determines whether their offspring are male or female? Is it the mom or the dad? Well, the mom always donates an X chromosome, so in no way does what the haploid genetic makeup of the mom's eggs, of the gamete from the female, in no way does that determine the gender of the offspring. It's all determined by whether-- let me just draw a bunch of-- dad's got a lot of sperm, and they're all racing towards the egg. And some of them have an X chromosome in them and some of them have a Y chromosome in them. And obviously they have others. And obviously if this guy up here wins the race. Or maybe I should say this girl. If she wins the race, then the fertilized egg will develop into a female. If this sperm wins the race, then the fertilized egg will develop into a male. And the reason why I said it's ironic is throughout history, and probably the most famous example of this is Henry the VIII. I mean it's not just the case with kings. It's probably true, because most of our civilization is male dominated, that you've had these men who are obsessed with producing a male heir to kind of take over the family name. And, in the case of Henry the VIII, take over a country. And they become very disappointed and they tend to blame their wives when the wives keep producing females, but it's all their fault. Henry the VIII, I mean the most famous case was with Ann Boleyn. I'm not an expert here, but the general notion is that he became upset with her that she wasn't producing a male heir. And then he found a reason to get her essentially decapitated, even though it was all his fault. He was maybe producing a lot more sperm that looked like that than was looking like this. He eventually does produce a male heir so he was-- and if we assume that it was his child-- then obviously he was producing some of these, but for the most part, it was all Henry the VIII's fault. So that's why I say there's a little bit of irony here. Is that the people doing the blame are the people to blame for the lack of a male heir. Now one question that might immediately pop up in your head is, Sal, is everything on these chromosomes related to just our sex-determining traits or are there other stuff on them? So let me draw some chromosomes. So let's say that's an X chromosome and this is a Y chromosome. Now the X chromosome, it does code for a lot more things, although it is kind of famously gene poor. It codes for on the order of 1,500 genes. And the Y chromosome, it's the most gene poor of all the chromosomes. It only codes for on the order of 78 genes. I just looked this up, but who knows if it's exactly 78. But what it tells you is it does very little other than determining what the gender is. And the way it determines that, it does have one gene on it called the SRY gene. You don't have to know that. SRY, that plays a role in the development of testes or the male sexual organ. So if you have this around, this gene right here can start coding for things that will eventually lead to the development of the testicles. And if you don't have that around, that won't happen, so you'll end up with a female. And I'm making gross oversimplifications here. But everything I've dealt with so far, OK, this clearly plays a role in determining sex. But you do have other traits on these genes. And the famous cases all deal with specific disorders. So, for example, color blindness. The genes, or the mutations I should say. So the mutations that cause color blindness. Red-green color blindness, which I did in green, which is maybe a little bit inappropriate. Color blindness and also hemophilia. This is an inability of your blood to clot. Actually, there's several types of hemophilia. But hemophilia is an inability for your blood to clot properly. And both of these are mutations on the X chromosome. And they're recessive mutations. So what does that mean? It means both of your X chromosomes have to have-- let's take the case for hemophilia-- both of your X chromosomes have to have the hemophilia mutation in order for you to show the phenotype of having hemophilia. So, for example, if there's a woman, and let's say this is her genotype. She has one regular X chromosome and then she has one X chromosome that has the-- I'll put a little superscript there for hemophilia-- she has the hemophilia mutation. She's just going to be a carrier. Her phenotype right here is going to be no hemophilia. She'll have no problem clotting her blood. The only way that a woman could be a hemophiliac is if she gets two versions of this, because this is a recessive mutation. Now this individual will have hemophilia. Now men, they only have one X chromosome. So for a man to exhibit hemophilia, to have this phenotype, he just needs it only on the one X chromosome he has. And then the other one's a Y chromosome. So this man will have hemophilia. So a natural question should be arising is, hey, you know this guy-- let's just say that this is a relatively infrequent mutation that arises on an X chromosome-- the question is who's more likely to have hemophilia? A male or a female? All else equal, who's more likely to have it? Well if this is a relatively infrequent allele, a female, in order to display it, has to get two versions of it. So let's say that the frequency of it-- and I looked it up before this video-- roughly they say between 1 in 5,000 to 10,000 men exhibit hemophilia. So let's say that the allele frequency of this is 1 in 7,000, the frequency of Xh, the hemophilia version of the X chromosome. And that's why 1 in 7,000 men display it, because it's completely determined whether-- there's a 1 in 7,000 chance that this X chromosome they get is the hemophilia version. Who cares what the Y chromosome they get is, cause that essentially doesn't code at all for the blood clotting factors and all of the things that drive hemophilia. Now, for a woman to get hemophilia, what has to happen? She has to have two X chromosomes with the mutation. Well the probability of each of them having the mutation is 1 in 7,000. So the probability of her having hemophilia is 1 in 7,000 times 1 in 7,000, or that's 1 in what, 49 million. So as you can imagine, the incidence of hemophilia in women is much lower than the incidence of hemophilia in men. And in general for any sex-linked trait, if it's recessive, if it's a recessive sex-linked trait, which means men, if they have it, they're going to show it, because they don't have another X chromosome to dominate it. Or for women to show it, she has to have both versions of it. The incidence in men is going to be, so let's say that m is the incidence in men. I'm spelling badly. Then the incidence in women will be what? You could view this as the allele frequency of that mutation on the X chromosome. So women have to get two versions of it. So the woman's frequency is m squared. And you might say, hey, that looks like a bigger number. I'm squaring it. But you have to remember that these numbers, the frequency is less than 1, so in the case of hemophilia, that was 1 in 7,000. So if you square 1 in 7,000, you get 1 in 49 million. Anyway, hopefully you found that interesting and now you know how we all become men and women. And even better you know whom to blame when some of these, I guess, male-focused parents are having trouble getting their son.