Additive Alleles: Definition & Characteristics

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

This lesson will provide a description and examples of one of the many factors that lead to genetic variation: additive alleles. Following the lesson, there will be a brief quiz so that you can see how well you understood the concepts.

Introducing Additive Alleles

Are all people either tall or short, fat or thin, very dark or very light? It seems like a silly question, but this is the way that many people think of genes and traits. Very few phenotypes, or observable traits, are all-or-nothing . For example, people don't just come in 2 or 3 colors, as we would if skin color was as simple as a single dominant or recessive gene. Instead, other factors come into play in determining our characteristics. One factor is additive alleles, which are alleles that contribute to most observable traits, such as height, weight, hair color, eye color, and complexion.

As you may know, an allele is a variation of a gene (a gene is sequence of DNA stored on a chromosome that codes for a trait, like eye color or diabetes). Since humans usually inherit one copy of each chromosome from their mother or father, most people have 2 alleles for every gene. In contrast, there can be many more additive alleles for a given gene. In fact, the number of copies of an additive allele determines if you will have an observable trait.

Figure 1: Alleles are different versions of a gene.
alleles

Let's further explore how the number of additive alleles contributes to observable traits.

Polygenic Traits are Additive

Most traits are polygenic, meaning more than one gene contributes to their phenotypes. In this case, an individual inherits multiple copies of each allele, rather than inheriting one copy of each allele, from each parent. So, when a trait is polygenic, the alleles are additive. The phenotype, or observable trait, will depend on the number of the allele you inherit--the more of the allele present, the more intense the associated trait will be.

It's important to note that polygenic traits and phenotypes cannot be distinctly categorized. Rather, they exist on a spectrum--there is no black or white, but instead black, white, and countless shades of grey in between. That explains why there are a wide array of hair, skin, and eye colors as well as countless shades of those colors.

Figure 3: Skin color depends on the number of alleles we inherit from our parents for a particular color.
skin tone is additive

Determining Characteristics: An Example

Have you ever wondered how people can have twins with two entirely different skin tones? Two people who have a medium complexion are capable of giving birth to children with a wide variety of skin tones, because skin color is the result of a combination of light- and dark-skin alleles. Their children can have a medium, darker, or lighter skin tone. It all depends on the number of each allele that a child inherits.

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