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Experimental cross performed by Thomas Hunt Morgan, illustrating the X-linked inheritance of white-eyed mutation in fruit flies[1]
Experimental cross performed by Thomas Hunt Morgan, illustrating the X-linked inheritance of white-eyed mutation in fruit flies[1]

Sex linkage is the patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). They are characteristically different from the autosomal forms of dominance and recessiveness as they are different depending on the sex of the individual.

Since humans have several times as many genes on the female X chromosome than on the male Y chromosome, X-linked traits are much more common than Y-linked traits. Additionally, there are more X-linked recessive conditions than X-linked dominant, and X-linked recessive conditions affect males much more commonly, due to males only having the one X chromosome required for the condition to present.

In humans, X-linked traits are inherited from a carrier or affected mother or from an affected father. In X-linked recessive conditions, a son born to an unaffected father and a carrier mother has a 50% chance of inheriting the mother's X chromosome carrying the mutant allele and presenting with the condition. A daughter on the other hand has a 50% chance of being a carrier, however a fraction of carriers may display a milder (or even full) form of the condition due to their body's normal X-inactivation process preferably inactivating a certain parent's X chromosome (the father's in this case), a phenomenon known as skewed X-inactivation. If the condition is dominant, or if the father is also affected, the daughter has a 50% chance of being affected, with an additional 50% chance of being a carrier in the second case. A son born to an affected father and a non-carrier mother will always be unaffected due to not inheriting the father's X chromosome. A daughter on the other hand will always be a carrier (some of which may present with symptoms due to aforementioned skewed X-inactivation), unless the condition is dominant, in which case she will always be affected. There are a few Y-linked traits; these are inherited by sons from their father and are always expressed.

The incidence of X-linked recessive conditions in females is the square of that in males: for example, if 1 in 20 males in a human population are red-green color blind, then 1 in 400 females in the population are expected to be color-blind (1/20)*(1/20).

The inheritance patterns are different in animals which use different sex-determination systems. In the ZW sex-determination system used by birds, the mammalian pattern is reversed, since the male is the homogametic sex (ZZ) and the female is heterogametic (ZW).

In classical genetics, a mating experiment called a reciprocal cross is performed to test if an animal's trait is sex-linked.

X dominant affected father.svg
X dominant affected mother.svg
X recessive carrier mother.svg
Illustration of some X-linked heredity outcomes (A) the affected father has one X-linked dominant allele, the mother is homozygous for the recessive allele: only daughters (all) will be affected. (B) the affected mother is heterozygous with one copy of the X-linked dominant allele: both daughters and sons will have 50% probability to be affected. (C) the heterozygous mother is called "carrier" because she has one copy of the recessive allele: sons will have 50% probability to be affected, 50% of unaffected daughters will become carriers like their mother.[2]

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  • ✪ Sex-linked traits | Biomolecules | MCAT | Khan Academy
  • ✪ Genetics part 6 sex linkage (sex linked inheritance)
  • ✪ Sex Linkage Practice Problems
  • ✪ Sex Linked Traits: Baldness and Hemophilia
  • ✪ Sex Chromosomes and Sex-Linked Traits


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.


X-linked dominant inheritance

An example pedigree chart of the inheritance of a sex-linked disorder
An example pedigree chart of the inheritance of a sex-linked disorder

Each child of a mother affected with an X-linked dominant trait has a 50% chance of inheriting the mutation and thus being affected with the disorder. If only the father is affected, 100% of the daughters will be affected, since they inherit their father's X chromosome, and 0% of the sons will be affected, since they inherit their father's Y chromosome.

There are less X-linked dominant conditions than X-linked recessive, because dominance in X-linkage requires the condition to present with only a fraction of the gene expression of autosomal dominance, since roughly half (or as many as 90% in some cases) of a particular parent's X chromosomes are inactivated in females.


X-linked recessive inheritance

Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder, although differences in X chromosome inactivation can lead to varying degrees of clinical expression in carrier females since some cells will express one X allele and some will express the other. All males possessing an X-linked recessive mutation will be affected, since males have only a single X chromosome and therefore have only one copy of X-linked genes. All offspring of a carrier female have a 50% chance of inheriting the mutation if the father does not carry the recessive allele. All female children of an affected father will be carriers (assuming the mother is not affected or a carrier), as daughters possess their father's X chromosome. If the mother is not a carrier, no male children of an affected father will be affected, as males only inherit their father's Y chromosome.



  • Various failures in the SRY genes

Sex-linked traits in other animals

Related terms

It is important to distinguish between sex-linked characters, which are controlled by genes on sex chromosomes, and two other categories.[5]

Sex-influenced traits

Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.[6] Even in a homozygous dominant or recessive female the condition may not be expressed fully. Example: baldness in humans.

Sex-limited traits

These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes.[6] Examples: female sterility in Drosophila; and many polymorphic characters in insects, especially in relation to mimicry. Closely linked genes on autosomes called "supergenes" are often responsible for the latter.[7][8][9]

See also


  1. ^ Morgan, Thomas Hunt 1919. The physical basis of heredity. Philadelphia: J.B. Lippincott Company.
  2. ^ Genetics home reference (2006), genetic conditions illustrations, National Library of Medicine.
  3. ^ Morgan T.H. 1910. Sex-limited inheritance in Drosophila. Science 32: 120-122
  4. ^ Doncaster L. & Raynor G.H. 1906. Breeding experiments with Lepidoptera. Proceedings of the Zoological Society of London. 1: 125-133
  5. ^ Zirkle, Conway (1946). The discovery of sex-influenced, sex limited and sex-linked heredity. In Ashley Montagu M.F. (ed) Studies in the history of science and learning offered in homage to George Sarton on the occasion of his sixtieth birthday. New York: Schuman, p167–194.
  6. ^ a b King R.C; Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press. ISBN 0-19-530761-5
  7. ^ Mallet J.; Joron M. (1999). "The evolution of diversity in warning color and mimicry: polymorphisms, shifting balance, and speciation". Annual Review of Ecology and Systematics. 30: 201–233. doi:10.1146/annurev.ecolsys.30.1.201.
  8. ^ Ford E. B. (1965) Genetic polymorphism. p17-25. MIT Press 1965.
  9. ^ Joron M, Papa R, Beltrán M, et al. (2006). "A conserved supergene locus controls colour pattern diversity in Heliconius butterflies". PLoS Biol. 4 (10): e303. doi:10.1371/journal.pbio.0040303. PMC 1570757. PMID 17002517.

This page was last edited on 25 September 2019, at 11:43
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