In humans, which condition is an example of overdominance?

The correct answer is that sickle cell anemia is an example of overdominance. In genetics, overdominance, or heterozygote advantage, refers to a situation where the heterozygous genotype (carrying two different alleles for a trait) has a higher fitness compared to either homozygous genotype (carrying two identical alleles).

In the case of sickle cell anemia, individuals who are heterozygous for the sickle cell allele (having one normal allele and one sickle cell allele) have a protective advantage against malaria, a disease that can be fatal but is widespread in regions where sickle cell trait is prevalent. While homozygous individuals for the sickle cell allele may suffer from the debilitating effects of sickle cell disease, those who are heterozygous can better survive in malaria-endemic regions, thus showcasing the benefits of having two different alleles.

Understanding overdominance in this context illustrates how genetic variation can provide evolutionary advantages in certain environments, which is not the case for the other conditions listed.