What Is a Polygenic Trait & What Are Some Examples of This?

In order to properly understand the definition of a polygenic trait, also known as a polygenic inheritance, you should probably have a solid foundation of some basic genetics terms so that it all comes together nicely.

The most crucial thing to the understanding of polygenic traits and genetic variance are definitions of traits, phenotypes, and genotypes as well as Mendelian inheritance.

Traits

You have probably heard the term “trait” in association with describing someone and this is close to the definition of the term. There are character traits that are associated with behavior and then there are appearance traits that are visible. In the genetics word, the word trait refers to a feature of an organism, such as hair color, eye color, skin color, and height.

Phenotype

This one you probably have not heard as frequently as a trait. The term “phenotype” refers to a collection of an organism’s outward characteristics. The color of the eyes, hair, and skin are all components of the phenotype. In terms of the trait, phenotype refers to the available trait forms. For example, hair color is a trait, while brown, black, blonde, or gray hair are all phenotypes. Speaking even more genetically, an organism’s phenotype is the outward or visible result of the genome. In other words, the collection of phenotypes is the result of the work of a person’s genes, which brings us to the next term, namely genotype.

Genotype

Just like phenotype, you probably have not heard this one so frequently either. Perhaps you have but, just in case you are not sure about it, the genotype is the entire collection of a person’s genes or the entire heritable genetic identity. In other words, genotype determines the set of visible traits or characteristics or an organism’s phenotype. Yet, in other words, an organism’s phenotype is the result of the genes as well as their actions as well as environmental factors (which we will not get into here). It should also be added that genes exist in types, also called alleles, that are found in the same place of a chromosome. Alleles are basically variant forms of genes coding for different observable phenotypes.

Mendel and Co.

The actual terminology dates back to 1911, when a Danish botanist, plant physiologist, and geneticist, Wilhelm Johannsen, used beans to demonstrate what we understand to be the laws of inheritance today. The actual distinction between genotype and phenotype, how they are interrelated, and how phenotypes are used to distinguish genotypes are based on both the studies of Johannsen as well as Gregor Mendel. Mendel was an Augustinian monk who conducted a series of pea plant experiments that lead to the establishment of the rules of heredity. His research on plants lead to our understanding of the rules of inheritance (also referred to as Mendelian Inheritance), which is why he is often referred to as the Father of Modern Genetics. Mendel’s research also led to the term “test cross,” which refers to crossing or breeding of two organisms in order to explore an organism’s genotype. This became a particularly relevant method as different alleles do not always result in equal phenotypes. In other words, looking at an organism, it is not known whether they are heterozygous or homozygous. In order to determine an organism’s genotype, or to genotype the organism, a test cross is performed between that and an organism that is known to be homozygous for a recessive trait. If any of the offspring display recessive traits, however, then the organism in question is heterozygous. Ifthe test cross results in offspring with the dominant phenotype, then the parent organism is homozygous dominant. These investigations, as well as Johannsen’s genotype-phenotype distinction, also lead to a better understanding of variation in organisms’ inherited traits and their evolution. It is thanks to them that we understand that a genotype is the result of both genetics as well as environment.

So, given all the aforementioned, just what are polygenic traits and how do they fit into the whole story?

Polygenic Traits

located close together on chromosomes or not

Typically, each trait is controlled by just one gene. However, there are some traits that are controlled by the work of several genes. These genes can either be located close together on chromosomes or not. They can even be on separate chromosomes. In other words, distance does not matter. Being informed after having read everything, you may be wondering just what type of inheritance polygenic traits follow. By virtue of there being several genes involved in one trait, polygenic traits do not follow Mendel’s pattern of inheritance. Rather than being measured discretely, they are represented as a range of continuous variation. Let’s use eye color, for example. The amount of blue in a person’s eye is difficult to quantify. The different hues of blue in the trait of eye color is a range rather than several distinct numbers, which is why polygenic traits are often expressed as a bell curve. Just like non-polygenic traits, polygenic traits also have recessive and dominant alleles. However, since several genes are involved in the phenotype, the final result is an amalgam of information which makes it difficult to impossible to tease out one gene’s impact on a polygenic trait.

Polygenic Trait Examples

Some examples of polygenic traits include characteristics that exist in this range of variables such as height, skin color, and eye color as well as hair color. An estimated 400 genes are related to height and each one of these genes plays a role in a person’s final height. You may think that this is an unreasonably high number. However, take into consideration the entire human body as well as the lengths of all the different body parts, and it becomes obvious why height should be the result of so many genes. Another example, skin color, is also the result of many genes. The main player in driving skin color is the pigment melanin that affects most of the skin phenotype. Skin color is darker with increasing amounts of melanin, which is also the body’s way of protecting itself from the damaging UV rays. Hence the darkness of the skin after sun exposure. The amount of melanin that is produced is the end result of several genes’ actions. The same is true for other phenotypes such as hair color and eye color.

Laura Day
 

>