What Is an Allele?

Alleles are gene copies that decide traits and characteristics

Alleles are copies of genes that influence hereditary characteristics. Each person inherits at least two alleles for a particular gene—one allele from each parent. They are also called allelomorphs.

A good example of how alleles are expressed is eye color; whether we have blue or brown eyes depends on the alleles that are passed down from our parents. Because they help determine what our bodies look like and how they're structured, alleles are considered an important part of the blueprint for all living organisms.

Understanding Alleles

Verywell / Jessica Olah


Alleles play a big role in determining our inherited traits, along with DNA and genes.

Deoxyribonucleic acid (DNA) is the hereditary material that humans and other living organisms get from each parent. It's technically a molecule that's responsible for carrying all of the necessary genetic information in the body’s cells. Half of a person's DNA comes from their mother, and the other half comes from their father.

Your DNA is organized into small parts called genes. Genes act as coded instructions to control how our bodies are built and influence what we look like. Experts estimate that humans have about 20,000 to 25,000 genes.

For most genes, one copy is inherited from the biological mother and one copy is inherited from the biological father (which we will refer to as simply the "mother" and "father" throughout).

The version of each gene that a parent passes down to their child is known as an allele. Alleles are located on chromosomes, which are the structures that hold our genes. Specifically, alleles influence the way our body’s cells work, determining traits and characteristics like skin pigmentation, hair and eye color, height, blood type, and much more. 

How It Works

The traits we end up inheriting from our parents depend on how the alleles interact with each other. The specific way that alleles are paired together are known as inheritance patterns, which make up all the variations in a person’s genetic traits.

Because alleles provide at least two sets of instructions for each gene, the body has to figure out which “roadmap” to follow, or in other words, which trait needs to be expressed.

Take eye color, for instance. A person’s eye color is a result of the alleles that were passed down from parent to child. Different combinations of alleles produce brown, blue, green, or hazel eye colors, though the last two are more unique than brown or blue eyes. Here's are two common scenarios that might occur:

  • If both parents contribute identical alleles for the eye color gene, they're known as homozygous. That means the instructions the alleles provide will be the same, so that eye color will appear. Homo- means same, and -zygous refers to the zygote that forms when a sperm fertilizes an egg.
  • If the parents each contribute different alleles for a gene, they're known as heterozygous, meaning the instructions won't match up, and the body will have to go in the direction of the stronger (or dominant) allele. Hetero- means different.

Here's where it can get a little tricky. An allele can be dominant or recessive. Dominant alleles express a trait, even if there is only one copy. Recessive alleles can only express themselves if there are two copies—one from each parent. And you've probably figured out by now that dominant alleles overrule recessive alleles.

For example, a trait like blue eyes is considered recessive, so it generally only appears when the blue eye alleles are the same from both parents. Brown eyes are considered dominant, so you only need that brown eye allele from one parent in order to have brown eyes.

Dominant and Recessive Traits

Brown eye color is a dominant trait, while blue eye color is a recessive trait. Green eye color is a mix of both and is dominant to blue but recessive to brown.

While two alleles make up the genotype, some traits, like eye color, have several alleles that influence the trait. This also includes blood type and hair color. New alleles arise in populations via mutation, and natural selection can also be an influence, deferring to some alleles over others

In fact, some biologists consider alleles to be so crucial to how humans have evolved that they define evolution as a change in allele frequencies within a population over time.


Alleles help decide almost everything about a living being. But even with a solid understanding of how alleles determine our traits and characteristics, genetics is still a complex field that scientists and researchers are learning more about every day.

It's worth mentioning that while it's possible to make fairly accurate predictions about what color eyes or hair your baby may have based on a combination of alleles, you can't always predict with absolute certainty which traits will appear.

Keep in mind that genetic combinations also depend on the "hidden" or recessive alleles that each parent may have.

Scientists originally thought that a single, simple inheritance pattern produced a person's eye color. But we now know that even dominant traits like brown eyes can be the result of multiple different allele combinations, and they can also disappear in one generation only to reemerge in a later generation.

In other words, because the way that alleles help determine eye color and many other traits is complex, genetic variations can sometimes produce unexpected results.

A Word From Verywell

While traits like eye color or hair color typically do not have any serious health conditions attached to them, if you have any questions about the way alleles can influence certain genetic diseases, you should feel comfortable addressing these concerns with your healthcare provider.

Armed with some background information about your family tree and medical history, a healthcare professional should be able to help you determine whether a specific genetic condition may run in your family and what it means for you and your loved ones.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  3. National Human Genome Research Institute. Gene.

  4. Liu F, Wollstein A, Hysi PG, et al. Digital quantification of human eye color highlights genetic association of three new loci. PLoS Genet. 2010;6(5):e1000934. doi:10.1371/journal.pgen.1000934

  5. Chen N, Juric I, Cosgrove EJ, Bowman R, Fitzpatrick JW, Schoech SJ, Clark AG, Coop G. Allele frequency dynamics in a pedigreed natural population. Proc Natl Acad Sci U S A. 2019;116(6):2158-2164. doi:10.1073/pnas.1813852116

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By Cristina Mutchler
Cristina Mutchler is an award-winning journalist with more than a decade of experience in national media, specializing in health and wellness content.