Edexcel iGCSE Biology-4.21B Codominance- Study Notes- New Syllabus
Edexcel iGCSE Biology-4.21B Codominance- Study Notes- New syllabus
Edexcel iGCSE Biology-4.21B Codominance- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
4.21B understand the meaning of the term codominance
Codominance
🌱 Introduction
In genetics, sometimes both alleles of a gene are equally expressed in the phenotype.
This situation is called codominance.
Unlike simple dominance (where one allele masks the other), in codominance both alleles are visible in the trait.
📖 Key Points about Codominance
- Definition: Codominance occurs when two different alleles of a gene are expressed equally and independently in a heterozygote.
- Result: The phenotype shows features of both alleles simultaneously, not a blend.
- Different from incomplete dominance:
- Incomplete dominance → blending of traits (e.g., red + white = pink flowers).
- Codominance → both traits appear side by side (e.g., red + white patches).
🌸 Classic Examples of Codominance
- Flower Color in Snapdragons: Red allele + White allele → plant shows red and white patches.
- Coat Color in Cattle: White allele + Red allele → roan coat (red and white hairs intermixed).
- Human ABO Blood Groups:
- IA and IB alleles are codominant.
- IA codes for antigen A, IB codes for antigen B.
- Genotype IAIB → both antigens appear → AB blood group.
📊 Summary Table
| Type of Inheritance | What Happens? | Example |
|---|---|---|
| Dominance | One allele masks the other | Brown eyes (B) over blue (b) |
| Incomplete dominance | Traits blend together | Red × White flower = Pink |
| Codominance | Both alleles expressed equally | AB blood group, roan cattle |
📌 Quick Recap
– Codominance = both alleles show up equally.
– Phenotype = side-by-side expression, not blending.
– Classic examples: AB blood group, roan cattle coats.
– Difference from incomplete dominance → codominance = coexistence, incomplete dominance = mix/blend.
✨ Trick: Codominance = Coexistence (both alleles show together).