IGCSE Biology glossary
Co-dominance
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
A pattern of inheritance in which both alleles of a gene are expressed equally in the phenotype of a heterozygote, rather than one being dominant over the other. For example, in sickle cell disease, a heterozygote (HbA HbS) produces both normal and sickle-shaped red blood cells.
What to include in an exam answer
In co-dominance, use two different CAPITAL letters for the alleles (e.g. CR and CW, not C and c). The heterozygote shows a blended or combined phenotype. Do not confuse with incomplete dominance.
Example in context
In a co-dominance cross of red flower (CRCR) and white flower (CWCW), what phenotype do the F1 offspring show? All F1 offspring are CRCW and show pink flowers (both red and white alleles are expressed).
Related syllabus topic
Inheritance