The sickle-cell allele is pleiotropic (i.e. ,it affects more than one phenotypic trait).Specifically,this allele affects oxygen delivery to tissues and affects an individual's susceptibility to malaria.Under conditions of low atmospheric oxygen availability,individuals heterozygous for this allele can experience life-threatening sickle-cell "crises." Such individuals remain less susceptible to malaria.Thus,pleiotropic genes/alleles such as this can help explain why

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Pleiotropic genes/alleles such as the sickle-cell allele affect multiple phenotypic traits, which can result in trade-offs or compromises in fitness. In this case, individuals with the sickle-cell allele have a decreased susceptibility to malaria, but are also at risk for life-threatening sickle-cell "crises" under low oxygen conditions. This illustrates the idea that adaptations are often compromises.

The Sickle-Cell Allele Is Pleiotropic (I