Codominance is when both alleles for a gene are equally strong and expressed simultaneously. This is different from incomplete dominance where traits blend. ABO blood grouping is an example. Codominance can result in interesting color and coat patterns in animals and plants. Cross-generational records are needed to determine dominance or recessiveness of a trait.
Codominance is a genetic trait in which both alleles for a gene are equally strong, resulting in both being expressed simultaneously. This is sometimes confused with incomplete dominance, a phenomenon where 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 co-dominantly express themselves.
This trait has important implications for a number of inherited characteristics. One of the most famous is the ABO blood grouping system. A person may inherit two alleles for blood A, resulting in blood type A, or two alleles for blood B, resulting in blood type B. If one of each is inherited, the person does not have a mixture of A and B, the patient has AB blood, expressing both alleles simultaneously and demonstrating codominance. Conversely, neither the A nor B alleles can be inherited, resulting in type O blood.
Understanding dominance and recession patterns is a key aspect of genetics. People involved in breeding organisms have to think about 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 an interesting color and coat pattern. People who raise chickens, for example, might cross black and white birds to develop barred or spotted chickens, since the alleles for these colors are codominant. Similarly, tab coats in cats are the result of codominance, caused by the simultaneous expression of two different alleles.
To determine whether a trait is dominant, recessive, or some variation thereof, people need access to cross-generational records 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 not fully dominant.
They may also find that one trait, such as yellow, is dominant, with that trait always expressing itself when a flower inherits one blue and one yellow allele. If so, the blue color pattern would be recessive, with a flower needing two blue alleles to appear blue. Crossing blue flowers would always result in 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.
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