Polygenic Inheritance of Traits Like Eye Color and Skin Color

Polygenic Traits Distribution

In polygenic inheritance, the genes contributing to a trait have equal influence and the alleles for the gene have an additive effect. Polygenic traits do not exhibit complete dominance as do Mendelian traits, but exhibit incomplete dominance. In incomplete dominance, one allele does not completely dominate or mask another. The phenotype is a mixture of the phenotypes inherited from the parent alleles. Environmental factors can also influence polygenic traits.

Polygenic traits tend to have a bell-shaped distribution in a population. Most individuals inherit various combinations of dominant and recessive alleles. These individuals fall in the middle range of the curve, which represents the average range for a particular trait. Individuals at the ends of the curve represent those who either inherit all dominant alleles (on one end) or those who inherit all recessive alleles (on the opposite end). Using height as an example, most people in a population fall in ​the middle of the curve and are average height. Those on one end of the curve are tall individuals and those on the opposite end are short individuals.

Eye Color

Eye color is an example of polygenic inheritance. This trait is thought to be influenced by up to 16 different genes. Eye color inheritance is complicated. It is determined by the amount of the brown color pigment melanin that a person has in the front part of the iris. Black and dark brown eyes have more melanin than hazel or green eyes. Blue eyes have no melanin in the iris. Two of the genes that influence eye color have been identified on chromosome 15 (OCA2 and HERC2). Several other genes that determine eye color also influence skin color and hair color.

Understanding that eye color is determined by a number of different genes, for this example, we will assume that it is determined by two genes. In this case, a cross between two individuals with light brown eyes (BbGg) would produce several different phenotype possibilities. In this example, the allele for black color (B) is dominant to the recessive blue color (b) for gene 1. For gene 2, the dark hue (G) is dominant and produces a green color. The lighter hue (g) is recessive and produces a light color. This cross would result in five basic phenotypes and nine genotypes.

• Black eyes: (BBGG)
• Dark Brown eyes: (BBGg), (BbGG)
• Light Brown eyes: (BbGg), (BBgg), (bbGG)
• Green eyes: (Bbgg), (bbGg)
• Blue eyes: (bbgg)

Having all dominant alleles results in black eye color. The presence of at least two dominant alleles produces the black or brown color. The presence of one dominant allele produces the green color, while having no dominant alleles results in blue eye color.

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Skin Color

Like eye color, skin color is an example of polygenic inheritance. This trait is determined by at least three genes and other genes are also thought to influence skin color. Skin color is determined by the amount of the dark color pigment melanin in the skin. The genes that determine skin color have two alleles each and are found on different chromosomes.

If we consider only the three genes that are known to influence skin color, each gene has one allele for dark skin color and one for light skin color. The allele for dark skin color (D) is dominant to the allele for light skin color (d). Skin color is determined by the number of dark alleles a person has. Individuals who inherit no dark alleles will have very light skin color, while those that inherit only dark alleles will have very dark skin color. Individuals who inherit different combinations of light and dark alleles will have phenotypes of varying skin shades. Those who inherit an even number of dark and light alleles will have a medium skin color. The more dark alleles inherited, the darker the skin color.

Polygenic Inheritance Key Takeaways

• In polygenic inheritance, traits are determined by multiple genes, or polygenes.
• Polygenic traits may express several different phenotypes, or displayed characteristics.
• Polygenic inheritance is a type of incomplete dominance inheritance, where the expressed phenotypes are a mixture of inherited traits.
• Polygenic traits have a bell-shaped distribution in a population with most individuals inheriting various combinations of alleles and falling within the middle range of the curve for a particular trait.
• Examples of polygenic traits include skin color, eye color, hair color, body shape, height, and weight.

Sources

• Barsh, Gregory S. "What Controls Variation in Human Skin Color?" PLoS Biology, vol. 1, no. 1, 2003, doi:10.1371/journal.pbio.0000027.
• "Is Eye Color Determined by Genetics?" U.S. National Library of Medicine, National Institutes of Health, May 2015, ghr.nlm.nih.gov/primer/traits/eyecolor.