Codominance

Codominance is an inheritance pattern in which both alleles are distinctly expressed in a heterozygote. Neither allele masks the other, so both contribute recognizable features to the phenotype.

Allele: An allele is one of the alternative versions of a gene at a particular genomic location. Codominance describes how different alleles at one gene are expressed together.
ABO blood group system: The ABO blood group system classifies blood by A and B antigens on red blood cells. In type AB, A and B alleles are both expressed as red-cell antigens.
Complete dominance: Complete dominance occurs when one allele determines the heterozygote’s phenotype and the other has no distinguishable effect. It contrasts with codominance, where both alleles make distinct contributions.
Punnett square: A Punnett square is a diagram for predicting possible offspring genotypes from parental gametes. It predicts codominant genotypes and their distinct heterozygous phenotypes.
Heterozygote: A heterozygote is an organism with two different alleles at a genetic locus. Codominance appears when the two alleles in a heterozygote are both expressed.
MN blood group system: The MN blood group system classifies red cells by the M and N forms of a membrane glycoprotein. Heterozygotes express both M and N forms, providing a clear codominant example.
Mendelian inheritance: Mendelian inheritance describes patterns of trait transmission explained by the segregation and assortment of alleles. Codominance modifies the simple dominant–recessive pattern often used to introduce Mendelian inheritance.
Blood type: Blood type is a classification based on inherited antigens on the surfaces of blood cells. AB blood type results from codominant expression of A and B alleles.
Gene expression: Gene expression is the process by which information in a gene produces a functional RNA or protein. Distinct expression of both alleles produces codominant traits.
Roan cattle: Roan cattle have coats containing a mixture of red and white hairs. The distinct red and white hairs illustrate expression of both coat-color alleles.

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