Polygenic Traits

Polygenic Traits Definition

Rather than being controlled by just one gene, polygenic traits are controlled by multiple genes. They may be controlled by genes located near each other or even on different chromosomes. Mendel’s inheritance pattern does not apply to polygenic traits because multiple genes are involved. 

As opposed to being measured discretely, they are often represented as a range of continuous variation. Height, skin color, eye color, and hair color are examples of polygenic traits.

Traits, Phenotypes, and Genotypes

In order to better understand polygenic traits, it is necessary to explain traits, phenotypes, and genotypes. A trait is any characteristic of an organism, such as its eye color. A phenotype is a set of physical characteristics of an organism. A person’s hair color, skin color, and eye color are all part of their phenotype. An available form of a trait can also be described by it. There are three possible phenotypes of eye color, such as brown, hazel, and blue.

An organism’s genotype is its genetic makeup. An allele is a particular form of a gene for a particular trait, for instance. A person’s genotype determines their phenotype; if they possess certain alleles, they will have a certain physical appearance, like brown eyes or blue eyes. A person’s phenotype is formed by all of the interactions between their genes, although it can also be affected by their environment (e.g. sun exposure).

Non-Mendelian Inheritance

It is impossible to explain polygenic traits solely by Mendelian inheritance. Whenever a gene controls a trait, and the traits are discrete, Mendelian inheritance is involved. Gregor Mendel, an Austrian monk and botanist who studied pea plants in the 19th century, is credited with its name. There were a number of traits in Mendel’s pea plants that showed either/or phenotypes. Flowers can be white or purple, short or tall, or wrinkly or smooth. 

Every trait was represented by a single gene, which had two alleles: dominant and recessive. In plants with two dominant alleles, or with one dominant allele and one recessive allele, the flowers are purple, while in plants with two recessive alleles, the flowers are white.

Polygenic traits have dominant and recessive alleles, but so many genes influence an organism’s phenotype for these traits that the final result is the result of many complex interactions. One gene’s effect on a polygenic trait can be difficult or impossible to determine. 

When charted, polygenic traits form a bell curve rather than a ratio, as single-gene traits do. The color of human skin, for instance, varies on a continuous gradient from light to dark, and it cannot be quantified; one can only compare one’s skin tone to others to get an idea of how light or dark it is. Although some people have extremely light or extremely dark skin, the majority of people fall somewhere in the middle.

Examples of Polygenic Traits

Height

It is estimated that there are over 400 genes associated with height, and all of these genes interact to determine a person’s phenotype. This is a large number, but it makes sense since height is a combination of many different body parts, including legs, torsos, and even necks. There is also a possibility that polygenic traits can be influenced by an organism’s environment. 

Lack of nutrition during childhood can stunt a person’s growth and make them smaller and shorter than they would otherwise be. Genetics determine 90% of a person’s adult height, while the environment affects 10%.

Although tall parents tend to have tall children, there is a wide variation in the height that each child can be. On other words, tall parents can also have short children, and vice versa. This is represented by the many data points shown for each averaged height, with bigger data points representing a larger number of people.

Skin Color

In humans, skin color is influenced by many things, but the pigment melanin influences most of a person’s phenotype. In general, the more melanin a person has, the darker their skin is. Albino people produce no melanin at all. The body creates more melanin to protect against the sun’s UV rays, which is why skin darkens after prolonged sun exposure. 

The amount and type of melanin that a person produces, such as eumelanin, pheomelanin, and neuromelanin, is controlled by multiple genes, and the different types of melanin interact to form the final phenotype. For example, people with red hair have more pheomelanin and often have a pinkish skin tone.

Eye Color

There are 2 major human eye color genes, OCA2 and HERC2, but at least 13 other genes also play a role. The colored part of a person’s eye is the iris. It is a muscle that changes the size of the pupil in order to change the amount of light that is absorbed by the retina. A person’s eye color is determined by the pigmentation of their irises, but also by the way the cells in their irises scatter light. 

As with skin color, eye color is affected by the presence of melanin. People with brown eyes have a lot of melanin, while people with blue eyes have low melanin in the front part of the iris that is visible. Green eyes are caused by multiple factors; they are the result of a light brown iris combined with a blue tone given by light scattering.

Related Biology Terms

• Gene – The basic unit of heredity; made up of DNA, it is transferred by parent to offspring and codes for a specific part of the offspring’s phenotype.
• Allele – A certain variant of a gene.
• Melanin – A pigment in skin, hair, and eyes that affects its color.
• Phenotype – Any part of an organism’s physical appearance.

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