Codominance (Edexcel IGCSE Biology)

• On occasion, both alleles within a genotype are expressed in the phenotype of an individual - this is known as codominance
• Inheritance of blood group is an example of codominance
  • There are three alleles of the gene governing this instead of the usual two
  • I represents the gene and the superscript A, B and O represent the alleles
  • Alleles I^A and I^B are codominant, but both are dominant to I^O
  • I^A results in the production of antigen A in the blood
  • I^B results in the production of antigen B in the blood
  • I^O results in no antigens being produced in the blood
• These three possible alleles can give us the following genotypes and phenotypes

Codominance in blood types table

Genetic diagrams for codominance

• We can use genetic diagrams to predict the outcome of crosses that involve codominant alleles:

‘Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O’

Punnett square shows the inheritance of Blood Group

Interpreting the genetic diagram

• The parent with blood group A has the genotype I^AI^O
• The parent with the blood group B has the genotype I^BI^O
• We know these are their genotypes (as opposed to both being homozygous) as they can produce a child with blood group O and so the child must have inherited an allele for group O from each parent
• Parents with these blood types have a 25% chance of producing a child with blood type O