If both parents have normal hemoglobin, what is the likelihood of their child being a carrier of sickle cell disease?

When both parents have normal hemoglobin, the likelihood of their child being a carrier of sickle cell disease is indeed 0%. This is because sickle cell disease is caused by a mutation in the HBB gene, and being a carrier of the disease means possessing one copy of the mutated gene and one copy of the normal gene (heterozygous).

If both parents have normal hemoglobin, they are homozygous for the normal version of the gene, which means they do not possess the mutated gene at all. Therefore, any child they have will inherit normal genes only from both parents, resulting in a child who also has normal hemoglobin and is not a carrier.

This understanding blocks the possibility of producing a child with the sickle cell allele, hence making the chance of the child being a carrier 0%.