IGCSE Biology / Genes and inheritance
When neither allele wins
Edexcel International GCSE Biology (4BI1), chapter 18: Genes and inheritance, specification point 3.21B. Free lesson.
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In codominance, both alleles are expressed in a heterozygote, so its appearance differs from either homozygote. This changes the phenotype ratio and is seen in flower colour and human blood groups.
What codominance means
Usually, one allele can hide the effect of another. In codominance, neither allele hides the other. Both are expressed in the heterozygote, an individual with two different alleles. This means the heterozygote has a different phenotype from either homozygote. For flower colour, the red allele and the white allele are both expressed when together. The flower is pink because both pigments are present, not because the pigments blend.
How to write and cross codominant alleles
Codominant alleles are written as capital letters with superscripts on a shared letter. For the flower example, use Cᴿ for red and Cᵂ for white. Crossing two heterozygous plants shows how the genotypes and phenotypes relate. Each parent can pass on either Cᴿ or Cᵂ. The offspring genotypes are CᴿCᴿ, CᴿCᵂ, CᴿCᵂ and CᵂCᵂ. CᴿCᴿ flowers are red, CᴿCᵂ flowers are pink, and CᵂCᵂ flowers are white.
How codominance works in blood groups
The blood group gene has three alleles: Iᴬ, Iᴮ and i. This is called multiple alleles. Although there are three possible alleles in the population, each person has only two, one on each chromosome of the pair. Iᴬ and Iᴮ are codominant with each other, and each is dominant to i. IᴬIᴬ or Iᴬi gives group A. Iᴬ is dominant to i. IᴮIᴮ or Iᴮi gives group B. Iᴮ is dominant to i. IᴬIᴮ gives group AB because both codominant alleles are expressed. ii gives group O because both alleles are recessive.
Working out possible blood groups in children
Start by writing each parent's genotype, then list the gametes each can make. A gamete carries one allele, so this shows which allele each parent can pass on.