Final answer:
The heterozygous sickle-cell genotype is common in malarial regions because it provides resistance to malaria, increasing survival and reproduction. This form of balanced polymorphism favors the maintenance of the sickle-cell allele in regions where malaria is prevalent, but not in areas without malaria, where the allele confers no benefit and can cause health issues.
Step-by-step explanation:
The heterozygous sickle-cell genotype is common in regions where malaria is prevalent because individuals with one copy of the sickle-cell allele (AS) exhibit resistance to malaria. This resistance increases their chances of survival and reproduction compared to individuals without the allele in malaria-endemic areas. As a result, the sickle-cell allele remains in the gene pool. The higher survival rate of heterozygotes in these areas is an example of balanced polymorphism, where two different versions of a gene are kept in balance because the heterozygote is at an advantage. In the case of sickle-cell anemia, heterozygotes have one normal hemoglobin allele (A) and one mutated allele (S), providing protection against malaria without the severe symptoms associated with having two mutated alleles (homozygous SS), which causes full-blown sickle-cell anemia with no malaria protection.
It is important to note that in non-malarial regions, the sickle-cell allele does not provide a survival advantage and can lead to health complications, even in heterozygous individuals. Therefore, outside of malarial regions, there is no natural selection favoring the maintenance of the sickle-cell allele within the population. The detrimental health effects of the sickle-cell allele in areas without malaria would lead to a decrease in its frequency over time if natural selection is the only factor considered.