Final answer:
A sickle-cell hemoglobin (HbS) curve is comparable to normal hemoglobin in an individual where blood CO₂ is elevated due to the decreased oxygen affinity and resultant shift of the curve to the right. This is similar to what occurs in sickle-cell anemia.
Step-by-step explanation:
The sickle-cell hemoglobin (HbS) curve reflects a reduced oxygen affinity compared to normal hemoglobin (HbA), underlining the impact of the genetic mutation that leads to sickle-cell anemia. The option comparable to the HbS curve would be 'Normal hemoglobin in an individual where blood CO₂ is elevated', which corresponds to option 5. This is because both scenarios involve a rightward shift in the oxygen dissociation curve, indicating a decreased affinity of hemoglobin for oxygen. This occurs in sickle-cell anemia due to the abnormal shape and rigidity of red blood cells and in elevated CO₂ conditions due to the Bohr effect. An increase in CO₂ levels leads to a decrease in pH, further promoting oxygen dissociation from hemoglobin and a shift of the curve to the right. Anemia in general might produce a similar effect, but it is the nature of sickle-cell anemia specifically, with its abnormal hemoglobin structure, that might resemble the effect of elevated CO₂ on normal hemoglobin's oxygen affinity.