Polygenic Traits Definition
Polygenic traits are traits that are controlled by multiple genes instead of just one. The genes that control them may be located near each other or even on separate chromosomes. Because multiple genes are involved, polygenic traits do not follow Mendel’s pattern of inheritance. Instead of being measured discretely, they are often represented as a range of continuous variation. Some examples of polygenic traits are height, skin color, eye color, and hair color.
Traits, Phenotypes, and Genotypes
To help understand polygenic traits, explanation of traits, phenotypes, and genotypes is necessary. A trait is any feature of an organism, such as eye color. A phenotype is the set of an organism’s physical characteristics. For example, a person’s hair color, skin color, and eye color are all part of their phenotype. It can also be used to describe the available forms of a trait. For example, eye color is a trait, while its possible phenotypes are brown, hazel, and blue.
A genotype is an organism’s genetic makeup. For example, a person may have a certain form of a gene, called an allele, for a trait. Genotype influences phenotype; if one has certain alleles, they will have a certain physical appearance, such as brown eyes or blue eyes. A person’s phenotype is the result of all of the interactions of their genes, although it can also be influenced by environment (e.g. sun tanning).
Polygenic traits are complex and unable to be explained by simple Mendelian inheritance alone. Mendelian inheritance is involved when one particular gene controls for a trait, and the traits are discrete. It is named after Gregor Mendel, an Austrian monk and botanist who studied pea plants in the 19th Century. Many of the traits in Mendel’s pea plants showed either/or phenotypes. For example, they could have white or purple flowers, short or tall, or have wrinkly seeds or smooth. This is because each trait was represented by only one gene which had two alleles: dominant and recessive. If a plant had two dominant alleles, or one dominant and one recessive allele, the flowers were purple, while if it had two recessive alleles, the flowers were white.
Polygenic traits also have dominant and recessive alleles, but so many genes play a role in an organism’s phenotype for these traits that the final result is the sum of many complex interactions. It can be hard or impossible to figure out one gene’s effect on a polygenic trait. Instead of being expressed in a ratio as single-gene traits are, polygenic traits are expressed continuously and usually form a bell curve when charted. For example, human skin color varies on a continuous gradient from light to dark, and it is not quantifiable; one’s skin color can only be compared to others for a sense of how light or dark his or her skin tone is. Some people have extremely light or extremely dark skin, but the majority of the world’s people do not, and fall somewhere in the middle.
This figure depicts a bell curve. For a trait like skin color, shade (light to dark) would be on the X (horizontal) axis, and proportion of population would be on the Y (vertical) axis. When data form a bell curve, they are said to show a normal distribution.
Examples of Polygenic Traits
Human height is controlled by many genes; in fact, there are over 400 genes related to height, and all of these genes interact to make up a person’s phenotype. This is a very large number, but it makes sense because height is a compilation of the lengths of many different body parts, such as leg bones, the torso, and even the neck. Polygenic traits can also be influenced by an organism’s environment. If a person gets inadequate nutrition during childhood, they can have stunted growth and end up smaller and shorter than they would otherwise. It is estimated that 90% of a person’s adult height is controlled by genetics, and 10% is affected by the environment.
This diagram shows the average offspring height based on the average height of both parents. 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.
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.
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.