Final answer:
The heterozygous sickle cell trait (Hb^s genotype) confers increased resistance to malaria. This evolutionary advantage is advantageous in regions with high malaria prevalence, whereas homozygous individuals suffer from sickle cell disease without added resistance. The correct option is 1.
Step-by-step explanation:
The sickle cell mutation, specifically the Hb^s genotype, presents a classic case of heterozygote advantage in biology. This condition known as sickle cell anemia arises due to a mutation in the beta-hemoglobin gene. Individuals who are homozygous for this mutation (ss) present with sickle cell disease, experiencing severe health issues. However, heterozygous individuals (AS) carry one normal hemoglobin allele and one mutated allele, leading to the production of some normal hemoglobin and some sickle hemoglobin. This mixture of hemoglobins offers a unique advantage to heterozygotes in regions where malaria is prevalent.
In response to the student's question, the direct answer is: 1) Increased resistance to malaria. Heterozygous individuals for the sickle cell trait have an enhanced protection against malaria, a feature that has been evolutionarily selected in regions where malaria is common, such as sub-Saharan Africa. This genetic adaptation is due to the fact that the altered shape and characteristics of the red blood cells in heterozygous individuals make it more difficult for the malaria parasite to infect and survive within the cells.
While the sickle cell trait does provide a significant advantage in terms of malaria resistance, it is important to note that having two copies of the mutated gene (being homozygous recessive) leads to sickle cell anemia, with no protective benefit against malaria and can cause serious health issues. Conversely, individuals who are homozygous dominant (SS) produce normal red blood cells and do not have this protection against malaria. Consequently, in countries where malaria is not endemic, the S allele of the sickle cell gene typically manifests as a purely disadvantageous trait, leading to health complications without any advantageous protection.