You may find the following diagram useful to help you understand how sickle haemoglobin is inherited. In all the following diagrams you will get the same possibilities if the genes in the mother and father are swapped over.
The lines coming in to each baby show that one gene has come from the mother and one gene has come from the father. In these diagrams the presence of the usual haemoglobin gene is shown by pink and the presence of the sickle haemoglobin gene is shown by blue.
The diagrams show the following combinations of parents and the types of children they can have:
- Sickle Cell Trait and Unaffected
- Sickle Cell Trait and Sickle Cell Trait
- Sickle Cell Trait and Sickle Cell Anaemia
- Sickle Cell Anaemia and Unaffected
If one parent has sickle cell trait (HbAS) and the other does not carry the sickle haemoglobin at all (HbAA) then none of the children will have sickle cell anaemia. There is a one in two (50%) chance that any given child will get one copy of the HbS gene and therefore have the sickle cell trait. It is equally likely that any given child will get two HbA genes and be completely unaffected.
If both parents have sickle cell trait (HbAS) there is a one in four (25%) chance that any given child could be born with sickle cell anaemia. There is also a one in four chance that aIf one parent has sickle cell anaemia (HbSS) and the other is completely unaffected (HbAA) then all the children will have sickle cell trait. None will have sickle cell anaemia. The parent who has sickle cell anaemia (HbSS) can only pass the sickle haemoglobin gene to each of their children. Any given child could be completely unaffected. There is a one in two (50%) chance that any given child will get the sickle cell trait.
If one parent has sickle cell trait (HbAS) and the other has sickle cell anaemia (HbSS) there is a one in two(50%) chance that any given child will get sickle cell trait and a one in two chance that any given child will get sickle cell anaemia. No children will be completely unaffected.
If one parent has sickle cell anaemia (HbSS) and the other is completely unaffected (HbAA) then all the children will have sickle cell trait. None will have sickle cell anaemia. The parent who has sickle cell anaemia (HbSS) can only pass the sickle haemoglobin gene to each of their children.