Codominance Calculators

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Codominance is a pattern of inheritance in which both alleles in a heterozygous individual are fully and simultaneously expressed, producing a phenotype in which both parental traits are distinctly visible — neither allele is dominant or recessive. This differs from incomplete dominance, where the heterozygote shows a blended intermediate phenotype. The most cited human example is the ABO blood type system, where individuals with IAIᴮ genotype are blood type AB, expressing both A and B antigens on red blood cells. Codominance is also seen in roan coat color in cattle and the MN blood group system.

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Codominance vs. Incomplete Dominance

Codominance: Both alleles fully expressed simultaneously — AB blood type expresses both A and B antigens. Roan cattle have both red and white hairs — both colors distinctly visible.

Incomplete dominance: A new intermediate phenotype is produced — red + white snapdragon → pink offspring. Neither red nor white is visible alone.

ABO Blood Type System

The ABO locus has three alleles: Iᴬ (A antigen), Iᴮ (B antigen), i (neither). IᴬIᴮ heterozygotes express both A and B antigens → type AB blood. This is codominance. Iᴬ and Iᴮ are each dominant over i, but codominant to each other. Genotypes: AA or Ai = type A; BB or Bi = type B; AB = type AB; ii = type O.

Other Codominance Examples

  • Roan cattle (shorthorn): RR = red; WW = white; RW = roan — distinct red and white hairs, not pink
  • MN blood group: M and N alleles — MN individuals express both M and N antigens
  • Sickle cell HbAS: Heterozygotes produce both normal HbA and sickle HbS simultaneously

Molecular Basis

Codominance occurs when both alleles encode distinct, independently functional proteins. Neither product suppresses the other. At the molecular level, codominance is the default — full dominance typically requires one allele's product to mask or substitute for the other's function, which is the special case.

Glossary

Codominance
A pattern of inheritance where both alleles in a heterozygote are fully and simultaneously expressed; neither is dominant; both parental traits are distinctly visible — not blended.
ABO Blood Type
The classic human codominance example; Iᴬ and Iᴮ alleles are codominant (IᴬIᴮ = type AB); both are dominant over i (ii = type O); determined by glycosyltransferases adding different sugars to H antigen.
Roan Coat Color
A codominant phenotype in shorthorn cattle where heterozygotes display intermixed red and white hairs (not pink blend); both hair colors are fully and simultaneously expressed.

Frequently Asked Questions

Codominance occurs when both alleles in a heterozygous individual are fully and simultaneously expressed. Neither allele is dominant or recessive — both produce their products and both are detectable in the phenotype. The classic example is ABO blood type: IAIᴮ genotype = type AB blood, expressing both A antigens and B antigens on red blood cells. The phenotype is not a blend — both parental traits are present simultaneously and distinctly.

Codominance: both parental traits are simultaneously and fully expressed in the heterozygote — roan cattle show distinct red and white hairs, or AB blood expresses both A and B antigens. Incomplete dominance: the heterozygote shows a new intermediate blended phenotype — pink snapdragons from red × white parents. Key distinction: codominance shows both originals at the same time; incomplete dominance produces a new, blended intermediate that is neither original.

The Iᴬ allele encodes a glycosyltransferase that adds N-acetylgalactosamine to H antigen, producing A antigen. The Iᴮ allele encodes a slightly different transferase that adds galactose instead, producing B antigen. In IᴬIᴮ individuals, both enzymes are produced, and both independently place their antigens on red blood cells. Neither enzyme interferes with the other — both are fully active. The result is simultaneous A and B antigen expression, producing type AB blood: classic codominance at the molecular level.

For two codominant alleles Cᴿ and Cᵂ: pure line cross Cᴿ Cᴿ × Cᵂ Cᵂ → all Cᴿ Cᵂ F1 (expressing both traits). F1 × F1 (Cᴿ Cᵂ × Cᴿ Cᵂ): 1/4 Cᴿ Cᴿ (trait 1 only) : 2/4 Cᴿ Cᵂ (both traits) : 1/4 Cᵂ Cᵂ (trait 2 only). Phenotype ratio = 1:2:1 — the same numerical ratio as incomplete dominance, but the heterozygote expresses both traits simultaneously rather than showing a new intermediate blend.