This diagram shows that both parents have one 'normal' haemoglobin gene (A) and one haemoglobin S gene (S) (as was in my case).

This means that the child has 1 in 4 (25%) chance of having 'normal' haemoglobin, a 2 in 4 (50%) chance of having one 'normal' and one haemoglobin S gene, therefore having Sickle Cell Trait, and a 1 in 4 chance of having Sickle Cell Anaemia.