Codominance and Multiple Alleles

This lesson covers codominance and multiple alleles — situations where inheritance patterns differ from simple Mendelian dominance — as required by the Edexcel A-Level Biology specification (9BI0, Topic 8).

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What Is Codominance?

Codominance occurs when both alleles in a heterozygote are fully expressed — neither is dominant over the other. The heterozygous phenotype is distinct from both homozygous phenotypes and shows features of both alleles simultaneously.

This is different from:

• Complete dominance — one allele completely masks the other
• Incomplete dominance — the heterozygous phenotype is a blend of the two homozygous phenotypes

Exam Tip: In codominance, both alleles are expressed simultaneously (e.g. both types of haemoglobin are present). In incomplete dominance, the heterozygote has an intermediate phenotype (e.g. red × white → pink). These are different concepts — do not confuse them.

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Notation for Codominant Alleles

When alleles are codominant, neither can be written as uppercase/lowercase. Instead, we use a superscript notation:

For a gene C:

• C^A and C^B are codominant alleles
• C^A C^A — homozygous for allele A
• C^B C^B — homozygous for allele B
• C^A C^B — heterozygous, expressing both alleles