A heterozygote is an organism that has two different forms of the same gene, with one being the dominant form and the other the recessive form. Genes are functional units that transmit hereditary information when organisms reproduce. They occur in pairs and have different forms, one dominant form and at least one recessive form, called alleles. Alleles can pair off in various ways such as one dominant with one recessive allele, two recessive alleles together, or two dominant alleles together. Dissimilar alleles occur in a heterozygote while either of the two similar pairs can occur in a homozygote.
Dominance is the ability of an allele to express its phenotype, or observable characteristics defined by its genetic code, while another allele’s phenotype is unexpressed. A common example of this is eye color in humans. Brown is a dominant trait while blue is recessive. If a person has one allele for brown eye color and one for blue, the person’s eyes will be brown because brown is dominant. Such a person is said to be heterozygous for eye color.
When a heterozygote exhibits evolutionary fitness greater than that of either homozygote, this phenomenon is known as a heterozygote advantage, or simply overdominance. Many experts think that evolutionary disadvantages can be preserved in the gene pool through this phenomenon. If having two copies of some allele is an evolutionary disadvantage, it would ordinarily be expected to disappear from the gene pool through natural selection. It can be preserved, however, if having one copy of it paired with another allele poses an evolutionary advantage.
The standard textbook example of this is sickle cell anemia. The recessive allele of a particular human gene causes sickle cell anemia, which is disadvantageous to an organism’s prospects for survival and reproduction. When coupled with another allele of the same gene, however, the gene pair confers on the organism resistance to a deadly disease called malaria. Such resistance can greatly improve an individual’s prospects for survival and reproduction in areas of the world where malaria is a concern.
Many other examples of heterozygote advantage are thought to exist, but other explanations for them have yet to be ruled out. Certain rats, for example, exhibit an advantage of pesticide resistance that appears to be related to a disadvantage of deficiency in vitamin K levels. Another example is a possible heterozygote advantage in female pigeons of lower microbial infection rates and higher rates of egg hatching.
The phenomenon opposite heterozygote advantage is, of course, known as heterozygote disadvantage, or underdominance. A particular species of grass has been found that is a possible example of underdominance. Heterozygous members of a natural population of this species have been found to produce fewer viable seeds than homozygotes of the same species.