We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

What is Codominance?

Mary McMahon
Updated: May 21, 2024

Codominance is a genetic trait where both of the alleles for a gene are equally strong, resulting in both being expressed simultaneously. This is sometimes confused with incomplete dominance, a phenomenon where the traits blend in their expression, as each trait expresses distinctly. In a simple example to differentiate the two, if a blue and yellow flower are crossed and a green flower develops, this is incomplete dominance. If the cross results in a yellow flower with blue spots, the traits for yellow and blue will express at the same time in codominance.

This trait has important implications for a number of inherited characteristics. One of the most famous is the ABO blood grouping system. A person can inherit two alleles for A blood, resulting in an A blood type, or two alleles for B blood, developing a B blood type. If one of each is inherited, the person doesn't have a blend of A and B, the patient has AB blood, expressing both alleles at once and demonstrating codominance. Conversely, neither A nor B alleles may be inherited, resulting in type O blood.

Understanding patterns of dominance and recession is a key aspect of genetics. People involved in breeding organisms must think about the patterns of genetic inheritance when selecting for specific desirable traits or attempting to eliminate bad ones. One result of codominance in animals and plants can be interesting color and coat patterns. People breeding chickens, for instance, might cross black and white birds to develop barred or spotted chickens, since the alleles for these colors are codominant. Likewise, tabby coats in cats are a result of codominance, caused by simultaneous expression of two different alleles.

In order to determine if a trait is dominant, recessive, or some variation thereof, people need access to records on multiple generations so they can follow the trait and see how it changes over time. In the example used in the first paragraph, researchers could track several generations of blue and yellow flowers and their offspring to see if the traits are codominant or incompletely dominant.

They might also find that one trait, like yellow, is dominant, with that trait always expressing when a flower inherits one blue and one yellow allele. In that case, the blue color pattern would be a recessive, with a flower needing two blue alleles to appear blue. Crossing blue flowers would always result in a blue offspring, but crossing a blue and yellow flower could result in a mix of blue and yellow offspring, unless the yellow flower had two yellow alleles, in which case all offspring would be yellow.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All The Science researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon943535 — On Apr 02, 2014

In humans, there are only three or four traits that are still thought to be simple dominant recessive with only two possible alleles.

By werterful — On Feb 02, 2014
Usually there are only two options or alleles for a gene; multiple alleles are when there are more than two. So for multiple alleles there has to be three or more options for the gene. This is different from what Gregor Mendel hypothesized. He theorized that there were just two alleles for every gene expression. However we now know that there can be more than two alleles for some genes. Of course each person can have no more than two alleles for every gene even though there may be more than two options.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

Learn more
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.