When two carriers have a child, there is a 1 in 4 risk that the child will have Sickle Cell Disorder. This risk is the same in each pregnancy. You cannot know in which pregnancy or in which order an affected child will come.
We inherit two genes for every characteristic, one from each parent. We all have two genes that control the haemoglobin in our red blood cells. The usual type of adult haemoglobin is called HbA. Most of us have two genes for HbA so we said to be AA. Being a Carrier of sickle cell means that you have ONE Normal GENE for the usual type of haemoglobin and one Abnormal for a type called HbS. You are AS. People who are AA or AS are completely healthy.
When a child is being formed,it inherits one gene for haemoglobin from each parent. From a parent who is AS it can receive either a gene for HbS. However, if the child inherits TWO GENES for HbSS he/she will have Sickle Cell Anaemia.
Statistical probabilities of inheriting sickle cell anemia.