The short answer
If both parents have sickle cell trait, each child has a 25% chance of sickle cell disease, a 50% chance of having the trait like their parents, and a 25% chance of inheriting neither sickle gene. The chances are the same in every pregnancy and the same for boys and girls. If one parent carries a different haemoglobin change instead — haemoglobin C or beta-thalassaemia, for example — a child can still have a form of sickle cell disease, so both parents' actual blood results matter.
Where the 25 / 50 / 25 comes from
Sickle cell trait means having one usual copy of the beta-globin gene and one sickle copy. When both parents have the trait, each can pass either copy on:
| From one parent | From the other | The child | Chance |
|---|---|---|---|
| Usual | Usual | Neither trait nor disease | 25% |
| Usual | Sickle | Sickle cell trait | 25% |
| Sickle | Usual | Sickle cell trait | 25% |
| Sickle | Sickle | Sickle cell disease | 25% |
The two middle rows are both trait, which is where the 50% comes from. It is the standard recessive pattern.
The chances for different parent combinations
| The two parents | Chance of sickle cell disease in each pregnancy | Other possible results |
|---|---|---|
| Trait (AS) + usual haemoglobin (AA) | 0% | 50% trait, 50% neither |
| Trait (AS) + trait (AS) | 25% | 50% trait, 25% neither |
| Trait (AS) + haemoglobin C trait (AC) | 25% (HbSC disease) | 25% AS, 25% AC, 25% neither |
| Trait (AS) + beta-thalassaemia trait | 25% (sickle beta-thalassaemia) | 25% AS, 25% thalassaemia trait, 25% neither |
| Sickle cell disease (SS) + usual (AA) | 0% | Every child has the trait |
| Sickle cell disease (SS) + trait (AS) | 50% | 50% trait |
The letters on the actual laboratory reports are what matter. There are several different beta-thalassaemia changes and other haemoglobin variants, so use this chart to work out which questions to ask — not as a substitute for both parents’ results.
Every pregnancy starts again
The table describes a single pregnancy, not a pattern that four children will conveniently follow.
Two carriers can have several children with sickle cell disease, or none at all. The genes passed to one child carry no memory of the last one.
When only one parent has the trait
If one parent has the trait and the other has two usual copies, each child has:
- a 50% chance of having the trait;
- a 50% chance of neither trait nor disease;
- no chance of sickle cell disease from that pairing.
That last line is only safe to say once the other parent’s blood result is actually known. Plenty of people carry a haemoglobin change without any symptoms and without ever being told.
Other haemoglobin traits change the answer
Sickle cell disease is not only “SS”. A child can also have a form of it by inheriting a sickle gene from one parent and a different beta-globin change from the other — haemoglobin C or beta-thalassaemia being the common ones.
So a partner result reading “not sickle trait” is not always the whole story. A blood test called haemoglobin electrophoresis, or something similar, looks at all the types of haemoglobin present. Genetic testing may be added when a result needs clarifying.
Trait is not disease
Most people with sickle cell trait never have the pain crises, anaemia or organ problems of sickle cell disease. Rare problems can occur with severe dehydration, very low oxygen, extreme exertion or high altitude, so it is worth knowing your result — but someone with the trait should not be described as having sickle cell disease.
What to take to an appointment
The useful documents are the actual laboratory results from both partners, not a family memory that somebody “has sickle”. Trait, disease and other haemoglobin changes get blurred together in conversation all the time, and a child’s risk depends on exactly which one each parent has.
Common questions
- Can two people with sickle cell trait have a child without it?
- Yes. Each child has a 25% chance of inheriting the usual copy from both parents, and so having neither the trait nor the disease.
- Our first child has sickle cell disease. Will the next one be healthy?
- There is no guarantee either way. The next pregnancy has the same 25% chance, because genes are passed on afresh each time. One affected child does not use up the risk.
- Can one parent with the trait have a child with sickle cell disease?
- Only if the other parent passes on another harmful beta-globin change — sickle, haemoglobin C, beta-thalassaemia or another variant. That is why both partners need a proper blood test.
- Is sickle cell trait the same as sickle cell disease?
- No. Trait means one sickle gene and one usual gene. Almost everyone with the trait is healthy, though rare problems can happen in extreme conditions such as severe dehydration or very low oxygen.
- Can it be tested for during pregnancy?
- Yes. If both parents carry a relevant change, a sickle cell and thalassaemia service can explain the testing available before or during pregnancy, and what each result could tell you.