Incomplete Dominance: Definition & Example

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Instructor: John Williams
Dominant/recessive inheritance is a common topic in genetics. Incomplete dominance is a naturally occurring phenomenon that may be present during the expression of this type of inheritance. This article describes incomplete dominance and goes over examples found in nature.

Definition of Incomplete Dominance

In nature, sexual reproduction is a process by which organisms increase genetic diversity in their offspring, where parents donate genetic material for specific traits (genes) to progeny (offspring). This is done so that the offspring will have a different combination of traits than their parents. Alternate versions of a gene are called alleles, and they determine what characteristics are exhibited in the individual.

In dominant and recessive heredity, dominant alleles are those that require only one copy to be displayed. Recessive alleles, however, require two alleles in order to show up in the organism. So under most circumstances, an individual with one dominant and one recessive allele (called a heterozygote) will only express the dominant version. However, this is not the case when it comes to incomplete dominance.

In the case of incomplete dominance, heterozygotes exhibit both alleles simultaneously, blended together. This is unlike codominance, where the traits are independently expressed together. Therefore, heterozygotes express entirely new phenotypes (physical expressions) that are not like the parent organisms. Incomplete dominance, while not the most common form of expression, is seen in many organisms, including plants, animals, and humans.

Examples of Incomplete Dominance

Have you ever seen pink roses? Pink roses are often the result of incomplete dominance. When red roses, which contain the dominant red allele, are mated with white roses, which is recessive, the offspring will be heterozygotes and will express a pink phenotype. Rather than express red or white, which is the parent phenotypes, the new phenotype is a blending of these two.

Andalusian chickens also exhibit incomplete dominance regarding feather color. In this particular breed, black chickens are dominant and white chickens are recessive. However, the heterozygotes express both, which creates offspring with a bluish-grey color. Once again, instead of expressing a phenotype like the parents (black or white), the heterozygote produces a new phenotype that blends the parent traits.

Sickle cell anemia is a disease that affects the formation of red blood cells in humans. The allele for sickled cells is recessive, and the allele for normal cells is dominant. Therefore, heterozygotes are called carriers and do not have the actual disease. However, they do produce some sickled cells and therefore, their blood phenotypes show both normal and affected red blood cells. This is an example of incomplete dominance in humans, as the normal blood type and sickled cell blood type are expressed simultaneously.

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