The short answer
If your brother or sister has an autosomal recessive condition and you do not, your chance of being a carrier is 2 in 3. Both your parents must be carriers, which gives each of their children a 25% / 50% / 25% split — affected, carrier, neither. But you already know you are not in the affected quarter. Take that quarter away and two of the three remaining possibilities are carriers.
Why two in three
Both of your parents must be carriers: each has one working copy of the gene and one changed copy. That gives their children four equally likely combinations.
| What you inherited | Result | Still possible for you? |
|---|---|---|
| Two working copies | Not a carrier | Yes |
| Working from mum, changed from dad | Healthy carrier | Yes |
| Changed from mum, working from dad | Healthy carrier | Yes |
| Two changed copies | Has the condition | No — you are healthy |
Three possibilities are left, and two of them are carriers. Two in three.
The 50% figure people reach for is the answer to a different question: the chance that a randomly chosen child of two carriers is a carrier, before you know how they turned out. Being healthy is information, and information changes the odds.
What it means for your own children
Your child can only be affected if you pass on a changed copy and your partner does too. So there are three things to multiply:
your chance of being a carrier × your partner’s chance of being a carrier × 1 in 4
If your partner is unrelated to you, and the condition is one that about 1 in 50 people carry:
2/3 × 1/50 × 1/4 ≈ 1 in 300
The middle number is the one that moves. How common a condition is varies hugely between conditions, and for several of them it varies sharply between populations — so where your partner’s family comes from genuinely belongs in the sum.
And if your partner is related to you, their chance is not the population figure at all. See first cousins having a baby.
The chances for everyone else in the family
Working outwards from your affected sibling, assuming everyone else is healthy:
| Relative | Chance of being a carrier |
|---|---|
| Each parent | Certain |
| You, a healthy brother or sister | 2 in 3 |
| Your child | 1 in 3 |
| A half-sibling through the carrier parent | 1 in 2 |
| An aunt or uncle on a carrier parent’s side | 1 in 2 |
| A first cousin through that aunt or uncle | 1 in 4 |
| A child of your affected sibling | Certain |
Your own child sits at 1 in 3: two thirds that you are a carrier, halved because you would only pass the changed copy on half of the time.
What a test does to the number
A test for the exact change found in your sibling is close to definitive. It looks for one specific thing already known to be in the family, so a negative result means far more than a negative on a general panel. If your affected relative has never had genetic testing, arranging it is the single most useful thing anyone in the family can do.
A general carrier panel finds most changes in a gene, but not all. Say a panel finds 9 out of 10 — then a negative result takes you from 2 in 3 down to roughly 1 in 6. A big drop, and not zero.
A negative result in your partner works the same way, and it moves the risk to a child far more than anything else available. From a starting point of 1 in 50, a good negative test leaves something like 1 in 500 — and the chance of an affected child drops to somewhere around 1 in 3,000.
Two situations that break the clean two in three
Rarely, an affected child’s two changed copies do not come one from each parent — the change may be new, or both copies may have come from the same parent. In that case one parent may not be a carrier at all.
And if you and your sibling are half-siblings, only the parent you share is implicated.
Both are uncommon. Both change the answer enough to be worth saying out loud rather than assuming away.
Common questions
- Why isn't it 50%?
- Because you already know you are healthy, and that rules out one of the four equally likely outcomes. Fifty per cent is the answer before that fact is taken into account.
- Does it change if several of my siblings are affected?
- No. One affected sibling already proves both parents are carriers; more affected siblings add nothing about you. You stay at two in three.
- Being a carrier — will it make me ill?
- For almost all of these conditions, no. Carriers are healthy. It matters for your children, not for you.
- What if my sibling's condition is X-linked rather than recessive?
- Then the arithmetic is different and depends on whether you are male or female — see X-linked recessive inheritance.
- Should I have a carrier test?
- It replaces a two-in-three guess with a yes or no, and it matters most if you are thinking about having children. If your family's exact gene change is already known, ask for a test for that specific change — it is far more informative than a general screen.
- Do I need to tell my relatives?
- Nobody can make you. But the information is about them too: aunts, uncles and cousins have meaningful chances of their own, and they generally cannot find that out unless somebody tells them the condition exists in the family.