Traits are passed from one generation to the next though genes. The genes for any trait have two alleles. When both the alleles for a trait are the same an organism is homozygous for the trait. If the two alleles differ, it makes an organism heterozygous for that trait.
When the presence of just one allele results in an advantage for an organism compared to either the presence of no alleles of the genes for that trait or the presence of both the alleles, it is known as heterozygote advantage.
Heterozygote advantage is the reason why the percentage of carriers of sickle cell anemia is very high among African-Americans and the percentage of carriers of cystic fibrosis very high among people of European descent. A single allele of sickle cell anemia makes people resistant from malaria, but does not result in any harmful effects due to sickle cell anemia. This makes people who live in areas where malaria is very common able to avoid dying from malaria and has resulted in an increase in the number of people who are carriers of sickle cell anemia. When two carriers have a child there is a 25% possibility that the child will have sickle cell anemia. The heterozygote advantage conferred is the reason why sickle cell anemia affects more people descendant from Asia and Africa where malaria is very common.
Similarly, a single allele for cystic fibrosis saves a person from dying due to cholera or tuberculosis. This has led to a increase in the percentage of people from Europe who are carriers of this illness but who do not suffer from the harmful effects of cystic fibrosis. When two carriers have children, there is a 25% chance that their offspring will suffer from the ill effects of cystic fibrosis.