Heterozygotes have two different forms of the same gene, with one dominant and one recessive allele. Heterozygote advantage occurs when a heterozygote has greater evolutionary fitness than both homozygotes, such as in sickle cell disease. Heterozygous disadvantage or underdominance can also occur, as seen in a species of grass.
A heterozygote is an organism that has two different forms of the same gene, one of which is the dominant form and the other the recessive form. Genes are functional units that transmit hereditary information when organisms reproduce. They come in pairs and have different forms, one dominant and at least one recessive, called alleles. Alleles can pair in various ways, such as a dominant allele with a recessive allele, two recessive alleles together, or two dominant alleles together. Dissimilar alleles occur in a heterozygote while one of two similar pairs can occur in a homozygote.
Dominance is the ability of one allele to express its phenotype, or observable characteristics defined by its genetic code, while the phenotype of another allele 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 one heterozygote exhibits greater evolutionary fitness than both homozygotes, this phenomenon is known as 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 an allele is an evolutionary disadvantage, one would normally expect it to disappear from the gene pool through natural selection. It can be preserved, however, if having one copy of it matched with another allele represents an evolutionary advantage.
The standard textbook example of this is sickle cell disease. The recessive allele of a particular human gene causes sickle cell disease, which is detrimental to an organism’s prospects for survival and reproduction. When paired with another allele of the same gene, however, the gene pair gives 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 problem.
Many other examples of heterozygous advantage are thought to exist, but other explanations have yet to be ruled out. Some rats, for example, show a pesticide resistance advantage that appears to correlate with a disadvantage in deficient vitamin K levels. Another example is a possible heterozygous advantage in female pigeons with lower rates of microbial infection and higher rates of hatching of the eggs.
The opposite phenomenon to heterozygous advantage is, of course, known as heterozygous disadvantage or underdominance. A particular species of grass has been found which is a possible example of subdominance. Heterozygous members of a natural population of this species have been found to produce fewer viable seeds than homozygous members of the same species.
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