In basic genetic inheritance, two different alleles are inherited from the two parents. One of these alleles is dominant and the other recessive. The dominant trait is expressed, seen in the offspring, while the recessive trait is hidden. This pattern of inheritance is explained in the article Understanding Dominant and Recessive Traits.
Incomplete dominance and Codominance are two other ways that genetic traits can be expressed. These two types of inheritance are often confusing to students, but there are some differences between the two.
What is Incomplete Dominance?
Incomplete dominance is when the two alleles inherited from the parents are neither dominant nor recessive, but blend together to give a physical trait that is somewhere between the two.
What is Codominance?
Codominance is quite similar to incomplete dominance in that neither parental trait is dominant nor recessive. However, with codominance, the two traits both appear in the offspring, often showing up in different parts of the plant or animal.
Examples of Incomplete Dominance and Codominance in Plants
In plants, snapdragons exhibit incomplete dominance for color traits. When a red snapdragon and a white snapdragon are crossed (mated), the color of the offspring is neither white nor red. Instead, the resulting snapdragon will be pink.
With codominance in flowers, on the other hand, the resulting offspring between red and white parents would not be pink. Instead, they would be red with white spots or white with red spots, the result of both colors being codominant.
Example of Incomplete Dominance and Codominance in Humans
The most well-studied example of incomplete dominance in humans occurs in the genes for curly hair. Inheriting a gene for curly hair from one parent and a gene for straight hair from the other parent will give a hair texture that is a blend of the two, wavy hair.
A codominant genetic trait in humans occurs with blood types. There are three different alleles for blood type: A, B and O. While the O type is actually the absence of either A or B proteins in the blood, the A and B blood types are codominant. If someone inherits one A allele and one B allele, they will not have a blood type that is in between those two, but will instead have a blood type AB, which expresses both A and B proteins in the blood.
Despite being similar, incomplete dominance and codominance are two different phenomena. Sometimes, the difference between them can only be seen at a molecular level. Nonetheless, they are distinct patterns of inheritance and understanding the differences between them can help students have a better grasp of how genes are expressed.
Sources:
Alberts, Bruce. Molecular Biology of the Cell. Garland Pub., 1989
Lewin, Benjamin. Genes IV. Oxford University Press., 1990
Bridget Coila - I'm a cell and molecular biologist, freelance writer and photographer currently living in Beijing, China. I'm fascinated by science, ...
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