Discuss what is meant by heterozygote advantage and use sickle-cell anemia as an example

When a single copy of a disease allele doesn't result in a disease but instead is good for the person or organism that carries it, we say that allele has a heterozygote advantage. In other words this occurs when heterozyhotes have increased fitness over both homozygotes.  A good example is sickcle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes.