Codominance

The phenotype displays the distinct contribution of each allele rather than hiding one allele or blending both into an intermediate form. This makes the organism’s observable traits especially informative about its genotype, because the presence of two different alleles can be reflected directly in the phenotype.

In the ABO system, an individual inheriting IA from one parent and IB from the other can have type AB blood. Both alleles contribute their corresponding A and B antigens, so the phenotype shows both contributions. This provides a clear biological example of how allelic interactions influence an observable human trait.

Codominance preserves the recognizable effects of both alleles, whereas incomplete dominance produces an intermediate phenotype between the parental forms. The distinction matters when connecting an observed trait to a genotype: a heterozygote displaying two separate contributions should not be interpreted as having a blended or partially expressed allele.

List the possible alleles contributed by each parent along the edges of the square, then combine one allele from each parent in every cell. Classify the resulting genotypes as homozygous or heterozygous and assign the corresponding phenotype. This process organizes expected offspring outcomes and helps compare possible allele combinations.

Pedigrees can help connect an individual’s visible phenotype with the alleles inherited from each parent. When a heterozygous phenotype displays both allele contributions, family patterns may reveal which relatives could transmit particular alleles. This supports analysis of inheritance across generations and helps distinguish codominant patterns from other allelic relationships.

The pattern is useful when researchers need to relate allelic differences to clearly distinguishable phenotypes. ABO blood groups illustrate this application in humans, while pedigree analysis and offspring predictions extend the approach to inheritance studies. Because both contributions can remain visible, codominance also helps explain variation within populations rather than treating traits as uniformly blended.