The short answer
In an X-linked dominant condition, one changed X is usually enough to cause symptoms in a daughter. In an X-linked recessive condition, a daughter with one changed X is usually called a carrier and is often well — though she can still have symptoms. In both patterns, a father passes his X to all of his daughters and none of his sons, and a mother with one changed X has a 50% chance of passing it to each child, whatever their sex.
The difference in one table
| X-linked dominant | X-linked recessive | |
|---|---|---|
| Does one changed X affect a daughter? | Usually yes | Often no symptoms or mild ones — but symptoms are possible |
| Are boys usually more severely affected? | Sometimes | Commonly |
| Can a father pass it to a son? | No | No |
| What about the daughters of an affected father? | All inherit it, and may be affected | All inherit it and are usually healthy carriers |
| What about a mother with one changed X? | Each child has a 50% chance of inheriting it | Each child has a 50% chance of inheriting it |
The labels describe what one changed copy usually does. They are not a promise that every woman with a recessive change is symptom-free, or that everyone with a dominant change is affected to the same degree.
The two rules that both patterns share
A father has one X and one Y. Every daughter gets his X; every son gets his Y. A mother has two X chromosomes and passes one of them, at random, to each child.
| Affected parent | Sons | Daughters |
|---|---|---|
| Father, mother unaffected | None inherit it | All inherit it |
| Mother with one changed X | 50% inherit it | 50% inherit it |
These are the chances of inheriting the gene change. They do not say how symptoms will turn out.
What it looks like in a family tree
The usual clues are:
- affected people in several generations in a row;
- both men and women affected;
- no father ever passing it to a son;
- every daughter of an affected father inheriting it;
- an affected mother passing it to about half her children.
The all-daughters-and-no-sons pattern from an affected father is the strongest clue of the lot. In a small family, chance can make a dominant condition look as though it mainly affects women. Chance cannot make a father give his X to a son.
Why women in the same family can be affected so differently
Early in development, every cell with two X chromosomes switches one of them off. Some cells end up using the X from one parent and some the other, which often softens the effect of a changed gene — plenty of cells are still running the working copy.
The balance is not the same in everyone. One woman can have obvious symptoms while a relative with exactly the same change has barely any. That is why “she seems fine” is not enough to rule out an X-linked change.
Why some conditions are worse in boys
A boy has one X chromosome. If its copy of the gene is changed, there is no second X to help out. For some conditions that means earlier or more severe symptoms; for others, an affected male pregnancy may not survive.
This is specific to each condition. “X-linked dominant” tells you how the gene travels through the family — not when symptoms will start or how serious they will be.
Why real families do not always fit the labels
The labels are useful for the first calculation, but real conditions are untidy. Women who carry an X-linked recessive condition can have symptoms. Women with an X-linked dominant change can be barely affected.
So the exact gene and the test result matter much more than the label. Start with the relative who actually has the condition: once their change is identified, everyone else can be tested for that one thing.
Then the first relatives to think about are an affected father’s daughters, because every one of them inherits his X. His sons are not at risk through him at all. If it is the mother who carries the change, every child — son or daughter — starts at the same 50%.
Common questions
- Can a father pass an X-linked condition to his son?
- No. A son gets his father's Y chromosome, not his X. A confirmed father-to-son transmission rules out an X-linked explanation.
- Are all daughters of an affected father affected?
- They all inherit his changed X. Whether that makes them ill, and how badly, depends on the particular condition.
- Does an affected mother pass it to half her children?
- Yes, on average. She has two X chromosomes and passes one of them on, so every pregnancy is a fresh 50% chance.
- Why are boys often more severely affected?
- A boy has only one X, so there is no second copy to make up for the changed one. In a few conditions, an affected male pregnancy may not survive — which can show up in a family history as repeated miscarriage alongside affected girls.
- How do I find out which pattern my family has?
- The condition's name and the exact gene change on a laboratory report will tell you. Testing an affected relative first identifies the family's change and turns everyone else's test into a straightforward yes or no.