The short answer
Duchenne muscular dystrophy is X-linked. If your mother carries your brother's DMD gene change, each daughter has a 50% chance of inheriting it. Where there is one affected boy and no previous family history, roughly two thirds of mothers have traditionally been estimated to carry the change — which puts a sister at a rough starting chance of about 1 in 3. Do not stop there: some cases are brand new in the child, and the family's exact change can usually be tested for directly. Women who do carry it also need heart checks of their own.
Find your next step from what the family already knows
| What you have now | The most useful next step |
|---|---|
| Your brother has Duchenne but no gene result | Get his DMD testing done or updated |
| His gene change is known; your mother is untested | Test your mother for that exact change |
| Your mother carries the change | Each daughter starts at 50% — ask for targeted testing |
| Your mother’s blood test is negative | Ask about mosaicism, and whether sisters should still be tested |
| A sister tests positive | Arrange heart follow-up, and discuss the 50% chance for each of her children |
| A sister tests negative for the known family change | She cannot pass that change on |
Following this order avoids using a broad carrier screen when the family already has one precise change to look for.
Why Duchenne nearly always affects boys
Duchenne muscular dystrophy is caused by a harmful change in the DMD gene, which sits on the X chromosome.
A boy has one X, from his mother, and one Y, from his father. If that X carries a Duchenne-causing change, there is no second copy to make enough of the muscle protein called dystrophin.
A girl has two X chromosomes. If one carries the change, the other usually prevents classic Duchenne — but she is not necessarily unaffected. Some carriers develop muscle symptoms, and all confirmed carriers need to know about the risk to the heart muscle.
Your chance depends on your mother’s result
If your mother carries the same DMD change as your brother:
| Her child | Chance of inheriting her changed X |
|---|---|
| A son | 50% — usually affected |
| A daughter | 50% — a carrier, with a health risk of her own |
So as her daughter, your starting chance is 50%. Feeling perfectly well and having no muscle symptoms does not reliably rule it out.
Where the one-in-three estimate comes from
When a boy is the first and only person in the family with Duchenne, older counselling rules estimated that about two thirds of mothers carried the change, and about one third of cases were brand new in the boy. A sister’s rough chance was therefore:
2/3 (chance mum is a carrier) × 1/2 (chance of inheriting her changed X) = 1/3
That shortcut was useful before modern testing existed. It is not a reason to stop at 1 in 3 today. The type of change, the possibility of mosaicism and the family’s actual results can all move the number a long way in either direction.
Test your brother’s gene change first
The clearest path is:
- identify the exact DMD change in your brother;
- test your mother for that change;
- test adult sisters and other women in the family for the same change.
If your brother never had complete genetic testing, updated testing may find a result now — and the exact change can also affect which Duchenne treatments he is eligible for, so it is worth doing for his sake as well.
What a negative result in your mother means
If the change is not found in your mother’s blood, it may have arisen for the first time in your brother. Your own chance is then much lower.
It is not always zero, though. A mother can carry the change in a small group of cells — including some egg cells — without it ever showing up in a blood test. That is called mosaicism, and it is also why a mother with a negative blood test still has a slightly raised chance of having another affected son.
Carrying it matters for your own health, not just your children’s
Women with a Duchenne-causing DMD change have an increased risk of cardiomyopathy — disease of the heart muscle. Some also have muscle weakness, cramps or fatigue. Symptoms can begin in adulthood, and how you felt as a child is no guide.
So a confirmed result leads to two separate conversations: what it means for any children you have, and a plan for monitoring your own heart.
If you do carry the family change, each of your children has a 50% chance of inheriting it. If testing shows you did not, you cannot pass that change on at all.
Common questions
- If my mother tests positive, what is my chance?
- Each daughter has a 50% chance of inheriting her changed X. A test for the exact family change can tell you whether you did.
- If my mother tests negative, am I definitely not a carrier?
- Your chance becomes much lower, but the result needs expert review. A change present in only a small share of her cells can be missed by an ordinary blood test, so your own test may still be worth having.
- Can women who carry Duchenne have health problems?
- Yes. Women with a DMD change have a raised risk of heart-muscle disease, and some have muscle weakness, cramps or fatigue. Anyone confirmed as a carrier should get advice about heart monitoring.
- Can a man with Duchenne pass it to his children?
- He passes his X to every daughter and his Y to every son — so all of his daughters inherit the change and none of his sons do.