The short answer
The gene sits on the X chromosome. A woman with one changed copy is usually a healthy carrier, because her other X covers for it. A boy with one changed copy is affected, because his only other sex chromosome is a Y. A carrier mother passes the changed X to half her children: each son has a 50% chance of being affected, and each daughter a 50% chance of being a carrier. An affected father passes his X to every daughter and to no son — so all his daughters are carriers and none of his sons are affected.
The chances, depending on which parent carries it
| The parents | Sons | Daughters |
|---|---|---|
| Carrier mother, unaffected father | 50% affected | 50% carriers |
| Affected father, mother not a carrier | None affected | All carriers |
| Carrier mother, affected father | 50% affected | 50% carriers, 50% affected |
The middle row is the one that surprises people. A man with an X-linked condition cannot pass it to his sons at all — he gives them his Y. His sons cannot pass it to their children either, so that branch of the family is finished with it.
Through his daughters it is a different story. Every one of them inherits his changed X, and the condition reappears in his grandsons.
What it looks like in a family tree
- Affected men, joined up through healthy women. A boy, his mother’s brother, his sister’s son: the condition travels along the female line without ever appearing in it.
- A father never passes it to a son. This is the clincher. One affected father with an affected son rules out X-linked inheritance completely and points to a dominant condition instead.
- Uncles on the mother’s side matter. When a boy is affected, the most informative person in the whole family is often his mother’s brother.
- Generations that look skipped, but are not. The gene passes through carrier women who are perfectly well, so the condition can disappear for a generation or two and come back — the commonest of the ways a condition skips a generation.
Carrier women are not always free of symptoms
Every cell in a woman’s body switches off one of her two X chromosomes early on, at random. So a carrier is a patchwork: some cells run the working copy, some the changed one. Usually that leaves plenty of working protein and she is completely well.
Sometimes the balance tips the wrong way and she has mild symptoms — occasionally the full condition. This matters in practice: a woman in the family described as “a bit affected but never diagnosed” should not be written down as unaffected without a second thought. For several X-linked conditions she is an important piece of evidence, and for some, such as Duchenne muscular dystrophy, a confirmed carrier needs health checks of her own.
X-linked dominant, in one paragraph
If one changed X is enough to make a woman ill too, the condition is called X-linked dominant. Affected women are then common — often outnumbering affected men. An affected father still passes it to all of his daughters and none of his sons. The difference is that the women are ill rather than healthy carriers. The comparison is set out in full on X-linked dominant vs recessive.
Why the numbers are hard to do in your head
X-linked family trees are where mental arithmetic gives up first, because the useful evidence is indirect.
A woman’s chance of being a carrier depends on her mother’s chance, which depends on how many healthy brothers and sons are in the family, on a grandfather nobody ever diagnosed, and on any old test result sitting in a file. Every healthy son she has already had is evidence against her being a carrier, and it adds up.
It is exactly the kind of arithmetic worth handing to a model instead of doing at the kitchen table.
Common questions
- Can a girl have an X-linked recessive condition?
- Yes, though it is uncommon. It takes an affected father and a carrier mother, or an unusual pattern in which her working X is switched off in most cells, or a girl who has only one X chromosome.
- My brother is affected. Am I a carrier?
- Your mother is very likely a carrier, which puts you at about 50% before anything else is taken into account. That number moves with the rest of the evidence: healthy sons of your own push it down, another affected relative on your mother's side pushes it up, and a genetic test settles it.
- Can my affected father pass the condition to my son?
- Not through his own sons — they get his Y, are unaffected, and cannot pass it on. Through his daughters it does continue: every daughter of an affected man is a carrier, and each of her sons has a 50% chance of being affected.
- How is this different from a recessive condition that is not X-linked?
- An autosomal recessive condition needs two changed copies in anyone, and affects boys and girls equally. An X-linked recessive condition needs only one in a boy. If the affected people in a family are nearly all male and all related through women, X-linked is the better explanation.
- What are some examples?
- Haemophilia A and B, Duchenne and Becker muscular dystrophy, red-green colour blindness and some forms of inherited vision loss.