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 inheritance, some characteristics are controlled by dominant and recessive alleles.
- However, not all alleles show this pattern.
- Sometimes, both alleles are expressed equally in the offspring.
- This pattern of inheritance is called codominance.
- In codominance, neither allele hides or masks the other.
🧬 What is Codominance?
- Codominance is a pattern of inheritance in which both alleles in a heterozygous individual are fully and equally expressed.
- As a result, both characteristics appear together in the phenotype.
Simple Definition: Codominance is when two different alleles are both expressed in the offspring, so neither allele is dominant or recessive over the other.
Characteristics of Codominance
- Both alleles are expressed equally.
- Neither allele masks the other.
- Both characteristics can be seen at the same time.
- Occurs in a heterozygous genotype.
- The offspring shows a phenotype that includes features of both alleles.
Example of Codominance: Red and White Coat Colour in Cattle
- This is the classic example of codominance.
Suppose:
- R = Red coat allele
- W = White coat allele
Neither allele is dominant over the other.
Possible Genotypes and Phenotypes
| Genotype | Phenotype |
|---|---|
| RR | Red coat |
| WW | White coat |
| RW | Roan coat |
What is a Roan Coat?
- A roan animal has both red and white hairs mixed together over its body.
- The coat is not pink or a blended colour.
- Individual red hairs and white hairs are both visible.
- This happens because both alleles are expressed equally.
- This makes roan cattle a classic example of codominance.
Why is Roan an Example of Codominance?
In the genotype RW:
- The red allele produces red hairs.
- The white allele produces white hairs.
- Since neither allele masks the other, both colours appear together.
Therefore, the animal has a roan coat.
🧩 Codominance vs Complete Dominance
| Feature | Complete Dominance | Codominance |
|---|---|---|
| Dominant allele | Masks the recessive allele | No allele masks the other |
| Expression | Only the dominant trait is seen | Both traits are seen together |
| Heterozygous phenotype | Same as dominant phenotype | Shows both characteristics |
| Example | Brown eyes over blue eyes | Red and white coat producing roan cattle |
🌟 Important Points
- Codominance occurs when both alleles are equally expressed.
- Neither allele is dominant or recessive.
- The heterozygous individual shows both characteristics at the same time.
- The phenotype is not a mixture, but a combination where both traits remain visible.
- Roan cattle are a common example of codominance.
📊 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).