6.2.6 - Codominance

• When both alleles of a gene in the genotype of a heterozygous individual contribute to that individual's phenotype, the alleles are described as codominant.
• The two alleles are responsible for two distinct and detectable gene products.
• The phenotype for heterozygotes is different to the genotype of the homozygotes.
• Coat colour in short horn cattle is an example of codominant inheritance - the gene for coat colour has two alleles:

  • Homozygotes with the Cr allele have red coats
  • Homozygotes with the Cw allele have white coats
  • Heterozygotes have a mix of red and white hairs - a roan coat.

  If red and white shorthorn cattle are interbred, all the offspring will be roan.

  If roan cattle are mated, the offspring will show all 3 phenotypes in the ratio 1 white: 2 roan: 1 red.
• The MN blood group system is controlled by a single gene with the two alleles Gm and Gn. The gene codes for a particular protein on the surface of erythrocytes - they each code for slightly different versions of the protein, but the alleles are codominant.

  Two heterozygous parents produce offspring with the 3 possible phenotypes in a ratio 1:2:1.
• Sickle Cell Anaemia:

  • Caused by an amino acid substitution in the B-Globin chain of haemoglobin.
  • The mutant allele is HBs; the normal all is Hbn.
  • The alleles are codominant. Heterozygotes have half and half of normal and abnormal haemoglobin.
  • Heterozygotes do not exhibit the disease - it is as if the abnormal allele is recessive.
• Codominant Inheritance in Plants:

  • Some flowers of camellia have red flowers and some have white flowers.
  • If these two types are crossed, the offspring will have red and white spotted flowers
  • Both alleles for petal pigment in this gene, Pr and Pw are expressed in the phenotypes of heterozygotes.