Final answer:
The type of selection related to sickle-cell and normal hemoglobin alleles in malaria regions is known as balanced selection. Heterozygous individuals have a survival advantage due to resistance to malaria, keeping the S allele in the gene pool despite its potential harmful effects in homozygosity.
Step-by-step explanation:
Selection in Sickle-Cell and Normal Hemoglobin Alleles
The term that could be used to describe the type of selection occurring with the sickle-cell and normal hemoglobin alleles among areas where malaria is present is balanced selection. This form of selection maintains the genetic diversity within a population by favoring the heterozygote individuals. In the case of sickle-cell anemia, individuals who are heterozygous for the sickle cell trait (carrying one sickle-cell allele and one normal allele) exhibit resistance to malaria, which increases their chances of survival and reproduction in regions where malaria is endemic. This advantageous effect keeps the S allele in the gene pool despite its potential harmful effects in the homozygous state, where two copies of the S allele lead to the severe symptoms of sickle-cell disease.Carriers of the sickle cell trait have a selective advantage in environments where malaria is prevalent. This explains why hemoglobin S, an abnormal hemoglobin variant, remains common in sub-Saharan Africa, the Middle East, and India, where it serves as an effective genetic adaptation against malaria. Conversely, individuals who are homozygous for the mutation suffer from the full effects of sickle-cell anemia, a condition that severely impacts health and life expectancy. Hence, the interplay between the environmental pressure of malaria and the genetic variations in hemoglobin illustrates how natural selection can shape the genetic landscape of human populations, maintaining alleles that confer a survival advantage in certain environments. However, without malaria, the frequency of the S allele may decrease over time because the selective pressure for its maintenance would be reduced.