The Biology Guide: Codominance vs. Incomplete Dominance

Waverly S.
4 days ago
4 min read

Olin, North Carolina

Biology and Genetics

Biology, the study of living organisms, can be a challenging subject, especially when it comes to heredity and genetics. These courses cover the processes through which certain traits are passed down through generations, and how offspring receive genes from their parents. Having a good understanding of heredity can be useful for a career in biology; even if biology is not your preferred field, it can be useful to know in your own life. Aside from mutations, all of the genes that you have come from your parents. Understanding heredity allows you to assess your risk in regards to certain genetic diseases, which could help you understand personal health outcomes- or even save your life. This article is meant to serve as a guide for students studying biology to help clarify the differences between two basic principles of heredity: codominance and incomplete dominance.

What is Dominance?

To understand these concepts, one must first understand what dominant and recessive alleles are. Typically, offspring will have two copies of any given gene, one from each of their parents. The two different versions of the same gene are called alleles. Dominant alleles are represented by uppercase letters, and recessive alleles are represented by lowercase letters. If an organism has one of each allele, it is called heterozygous, and if it has two copies of one allele, it is called homozygous. Genotype refers to the set of alleles that an organism has, while phenotype refers to the traits that an organism has. A dominant allele is a gene that will always be expressed as a trait if it is present, even if an organism only has one dominant allele present. However, a recessive allele will only present as a trait if there are only recessive alleles present. For example, imagine that the dominant allele in a dog breed is expressed as black fur and the recessive allele is expressed as white fur. In the case of complete dominance, if one parent, who had one dominant allele and one recessive allele, reproduced with a parent with two recessive alleles, there would be two possibilities for the offspring. If the offspring inherited the dominant allele from the heterozygous black parent, then it would have black fur. However, if it inherited the recessive allele from the heterozygous black parent, it would have white fur. Since the white parent has two recessive alleles, the offspring could only inherit a recessive allele from that parent.

Punnett square showing dog coat color genetics. Top: black dog with "BB", white dog with "bb". Squares: Bb black dogs, bb white dogs.

Codominance

Sometimes, one allele is not dominant over the other. When there is no single dominant allele, both traits are expressed equally when an organism has both alleles, which is called codominance. This can be represented in two ways: by just using regular uppercase and lowercase letters, or by using uppercase letters with a superscript, both of which will be displayed below. The method of representing codominance with a superscript is much more common, however, as there is no recessive allele, as a lowercase letter would imply in most cases. In the case of a heterozygous dog mating with a homozygous recessive dog, there are two possibilities for the offspring: black and white fur, or white fur. Note that if a dog ended up with two alleles that are expressed as black fur, the dog would have black fur.

Punnett square showing dog coat color genetics. Top: black and dog with "BDBW", white dog with "BWBW". Squares: BDBW black and white dogs, BWBW white dogs. — Codominance is typically represented with superscript letters.

Punnett square showing dog coat color genetics. Top: black and white dog with "BB", white dog with "bb". Squares: Bb black and white dogs, bb white dogs. — Remember that lowercase letters do not represent recessive traits when it comes to codominance, and that this is an uncommon way to represent the phenomenon.

Incomplete Dominance

Sometimes, neither allele can fully dominate the other, and neither allele can be completely expressed as a trait. This is called incomplete dominance, and when an organism has one of each allele, a blend of both traits is expressed, but neither trait is expressed independently. Once again, this can be represented through pairs of uppercase and lowercase letters or as capitalized letters with subscripts, but the method not using lowercase letters is far more common, as neither trait is recessive. In the case of a heterozygous dog reproducing with a homozygous recessive dog, the two possibilities for the offspring would be grey and white. Keep in mind that if a dog received two dominant alleles in this situation, it would have black fur.

Punnett square showing dog coat color genetics. Top: grey dog with "BDBW", white dog with "BWBW". Squares: BDBW grey dogs, BWBW white dogs. — Incomplete dominance is typically represented by superscript letters.

Punnett square showing dog coat color genetics. Top: grey dog with "Bb", white dog with "bb". Squares: Bb grey dogs, bb white dogs. — Remember that lowercase letters do not represent recessive traits when it comes to incomplete dominance, and that this is a uncommon way to represent the phenomenon.

Now, if you would like to check your understanding of this topic, try answering the questions below.

Check Your Understanding:

What is the difference between codominance and incomplete dominance?
If a homozygous dominant black dog reproduced with a homozygous recessive white dog, and the trait for fur color was affected by codominance, what would the offspring look like?
For practice, try making your own Punnett square. Choose two organisms and a trait that they could pass on to their offspring. Then, decide if that trait, for the purposes of your example, will be affected by dominance, codominance, or incomplete dominance. Try checking it with a friend or teacher when you are finished.

For more review, consider watching the videos linked below: