F. Heterozygous Advantage. Sickle-Cell Anemia is a good example. Thus the 6th amino acid is valine instead of glutamic acid. This changes the structural properties of the hemoglobin and causes the red blood cells with two copies of the allele to be sickle shaped and inefficient carrying oxygen. Homozygous recessive can be fatal without good medical care.
Heterozygous individuals have red blood cells that are only slightly effected.
They still work reasonably well carrying oxygen and they are less easily invaded by malaria parasites conferring a fitness advantage in the face of a high incidence of the disease. V. Costs of Inbreeding. Note that many recessive alleles when expressed can be deleterious reducing fitness. Anything that promotes their expression will be selected against and behaviors that prevent their expression will be favored. Most individuals have a few recessive alleles in the heterozygous condition that would be deleterious or even lethal if expressed in the homozygous condition.