Every characteristic of your body is controlled by “genes”, Genes are parent in every cell of your body. You have two of every kind of gene, one passed on form your mother, the other passed on form your father. You have two genes that control how haemoglobin is made on each of your red blood cells.
“Normal” people have two normal genes for haemoglobin. Healthy carriers of B –thalassaemia trait have one normal gene for haemoglobin and one altered one: they are halthy because one gene is working well. Since one gente is ingerited trom each parent, al least one of their parents must be a carrier.
People with B-thalassaemia major have two altered genes for haemoglobin, one inherited from each parent, so both their parents must be carriers.
Now let us consider three sorts of married couples:
- Both parents are “normal”.
They cannot possibly pass on thalassaemia trait or thalassaemia major of their children.
- One parent has thalassaemia trait and one is “ normal”
All the children must inherit a normal gene form the “normal” parent. However, they may inherit a normal or a thalassaemia gene from the carrier parent. For each child there is a one in two (50%) chance of inheriting the thalassaemia gene from the carrier parent: it this happens the child will be completely normal. None of this couple’s children can have thalassaemia major.
- Both the parents are B-thalassaemia carriers: i.e. they are a “couple at risk.”
This is why couples of carriers have a one in four( 25%) chance in each pregnancy of having a child with thalassaemia major; a one in two (50%) chance of having a child with thalassaemia trait; and a one in four (25%) chance that the child will inherit a normal gene from both parents, and so will be completely normal.
It is possible to test the foetus during pregnancy, to see if it has thalassaemia major. Then the parents can decide whether or not to keep the pregnancy. This is called parental diagnosis. To do this test, it is necessary to study blood from both parents, to find out exactly which type of thalassaemia gene they carry.