This diagram shows that one parent carries one 'normal' haemoglobin gene and one Sickle gene, and the other carries both Sickle genes.

This means that the child will have a 2 in 4 (50%) chance of having one 'normal' haemoglobin gene and one haemoglobin S gene, and a 2 in 4 (50%) chance of having both haemoglobin S genes.