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Polygenic Traits and Pleiotropy

Polygenic Traits and Pleiotropy

Author: Amanda Soderlind

Understand how genes interact with one another and their environment

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polygenic Traits and Pleiotropy

Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on pleiotrophy and polygenic traits.

Today we are going to be discussing how different traits can be expressed, and the way in which a trait is expressed can have an effect on a person's genotype or phenotype.

We're going to start by discussing what polygenic traits are. Polygenic traits are traits that are expressed as the combined expression of multiple genes. What this means is that polygenic traits are traits that are not determined by just one single gene. And in fact, most of the traits that we express are actually determined by many genes. They're polygenic traits. They're actually not very many traits that are determined by one single gene.

Let's use eye color as an example here. The color of your eye is actually determined by many genes. It's determined by multiple genes. It's not just determined by one single gene. So that's just one example. Think of traits such as your hair color, your skin color, your height. All of those are examples of polygenic traits because they are determined by multiple genes, and not just one single gene.

Polygenic traits can show continuous variation within a population. Height is a good example of a polygenic trait, as well, because within a given population, we could have a wide range of continuous differences of that trait. And height is also a multi-factorial trait. What this means is that it's determined by multiple factors. Height can be determined by a person's genes, for example, but it can also be determined by a person's environment. So your genes can help determine how tall you will be, but environmental factors, such as nutrition for example, can also play a role in a person's height, in their development. So height is also considered a multi-factorial trait, in addition to being a trait that shows continuous variation.

Penetrance is another term that we're going to talk about today. Penetrance is the varying degree to which someone expresses a trait that's associated with an allele. So basically what this means is that some people with inherited alleles for trait might have varying phenotypes. We're going to discuss incomplete penetrance, and a trait that's completely penetrant.

Let's start by talking about a trait that completely penetrate. Cystic fibrosis is an example of a trait that would be completely penetrate. What this means is that 100% of people who are homozygous recessive will have cystic fibrosis.

Polydactyly would be an example of a trait that would be incompletely penetrant. Polydactyly relates to the number of digits that a person has. Some people who have polydactyly, who carry the genes for polydactyly might have the normal 10 fingers. Some people who have that trait might have more than 10 fingers. So there's this varying degree to which someone expresses that trait. Some people would have the normal 10, some people may have more. So we have a varying degree to which someone expresses that trait, who has the alleles for polydactyly.

Pleiotrophy is the third term, the last term that we're going to talk about here today. Pleiotrophy is the expression of one gene that affects multiple traits. So we have one gene affecting multiple traits. An example of this is sickle cell anemia. So the gene for sickle cell anemia produces various affects throughout the body. So this one gene for sickle cell anemia produces multiple affects throughout the body. It can affect the way the blood carries oxygen. It can affect other internal organs, et cetera. So this one gene is having multiple affects on the body. So that's an example of pleiotrophy.

This lesson has been an overview on pleiotrophy and polygenic traits.

Terms to Know
Continuous Variation

Variation of a trait that shows up in a population of people, body height for instance.

Multifactorial Trait

A gene that is partially controlled by genetics and partially controlled by the environment. For example, body height is genetic but nutrition growing up also influences height.


The degree to which an inherited allele is expressed in the phenotype. An example would be familial breast cancer due to mutations in the BRCA1 gene. Of the people who inherit such a mutation, 80% will develop breast cancer over their lifetime. The penetrance of this allele is 80%.


When one gene influences multiple phenotypic traits. An example is sickle-cell anemia: a mutation in one of the hemoglobin genes results in phenotypic changes in the blood, the joints, etc.

Polygenic Trait

A trait that is determined by several genes.

Sickle-cell Anemia

An example of pleiotropy, a person who inherits the mutated HBs gene will contract sickle-cell anemia, which damages erythrocytes, the spleen, and many other organs of the body.