Final answer:
The difference between alleles A and S for the HBB gene is a single nucleotide change, causing a different amino acid to be incorporated into the protein, which can lead to the condition called sickle cell anemia.
Step-by-step explanation:
The difference in the DNA sequence between alleles A and S for the HBB gene for hemoglobin is represented by option D, there is a difference in a single nucleotide in the gene. The HBB gene encodes the beta chain of hemoglobin and a specific point mutation in this gene can lead to sickle cell anemia.
This mutation involves a single base change in the DNA sequence, where the amino acid glutamic acid in the beta chain is replaced by valine at the sixth position of the chain, due to a single nucleotide substitution (adenine to thymine). Even though each hemoglobin molecule is made up of approximately 600 amino acids encoded by three nucleotides each, it's this single base change that results in the dramatic structural and functional differences in the hemoglobin molecule, leading to the condition known as sickle cell anemia.
The remarkable aspect of this genetic variation is that it shows how a minuscule alteration in the DNA sequence can have a large impact on an individual's health. As a comparison, this change would be 1 in 1800 bases for the approximately 600 amino acids coded by the gene.