Exceptions to Mendel's Principles > Co-Dominance

Co-dominance is the pattern of inheritance in which both alleles of a gene express themselves equally and simultaneously in the heterozygous condition, so both parental traits appear side by side in the phenotype.

1. Both alleles in a heterozygote are active and functional
2. Each allele produces its own distinct protein/sugar on the cell surface
3. Neither protein interferes with the other - both work independently
4. Result: both parental traits appear in the same individual

The Gene:

Blood groups in humans are controlled by gene I, which has three alleles - Iᴬ, Iᴮ, and i​

• Iᴬ → produces Sugar A on red blood cell (RBC) surface
• Iᴮ → produces Sugar B on RBC surface
• i → produces no sugar

Dominance Relationships:

Why is IᴬIᴮ → AB an example of co-dominance?

Because when Iᴬ and Iᴮ are present together, "they both express their own types of sugars" — so the RBC surface has both Antigen A and Antigen B at the same time. Neither allele dominates the other.​

Full Genotype–Phenotype Table

Total Genotypes = 6 | Total Phenotypes = 4 (A, B, AB, O)

Parents (P Generation):

F₁ Generation - Cross: RR × WW

Punnett Square (P):

Result → All F₁ = RW = Roan

Roan coat = red hairs AND white hairs growing side by side on the same animal.
This is NOT a blended pink colour. Both parental hair colours are visible distinctly.​

F₂ Generation - Cross: RW × RW

Punnett Square (F₁ × F₁):

F₂ Genotypic ratio = Phenotypic ratio = 1 : 2 : 1
(This is a key feature of co-dominance — both ratios are identical)

Representation of co-dominance in cattle

• Co-dominance - Both alleles of an allelomorphic pair express themselves equally in F₁ hybrids; neither allele is dominant or recessive over the other.
• Example - Red cattle (RR) × White cattle (WW) → F₁ hybrids are Roan (RW); roan coat has a mixture of red and white hair - both traits expressed equally.
• F₂ Generation - Selfing of F₁ (RW × RW) gives:
  Genotypic ratio - 1RR : 2RW : 1WW
  Phenotypic ratio - 1 Red : 2 Roan : 1 White
• In co-dominance, genotypic and phenotypic ratios are identical (1:2:1); key difference from incomplete dominance is that both alleles are fully expressed, not partially.