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.

• 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.