Why is the sickle-cell allele described as adaptive in regions where malaria is common?

1. Sickle-cell anemia is a genetic condition in which round red blood cells take a crescent shape and clog blood vessels. 2. Carriers have only one copy of the sickle-cell gene, do not develop full anemia, and show symptoms only under severe dehydration or oxygen deprivation. 3. Carriers are thought to be immune from malaria, because changes in blood chemistry and immune function stop the parasite. 4. That protection belongs to carriers, not to those with two copies, who have the full disease; option A states the single most common error on this topic. 5. In the chapter's narrative the carrier survives malaria and reproduces while the non-carrier dies, which is what makes the allele adaptive in malarial regions and costly elsewhere. _Source: OpenStax Psychology (1st ed., CC BY 4.0), Ch 3 "Biopsychology", §3.1 Human Genetics_