Edexcel A Level (IAL) Biology -3.21 Polygenic Inheritance & Continuous Variation- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.21 Polygenic Inheritance & Continuous Variation- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.21 Polygenic Inheritance & Continuous Variation- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.21 understand how some phenotypes are affected by multiple alleles for the same gene, or by polygenic inheritance, as well as the environment, and how polygenic inheritance can give rise to phenotypes that show continuous variation
Polygenic Inheritance & Continuous Variation
🌱 Introduction
Most traits (like height, skin color, or eye color) are not controlled by just one gene. Instead, they are influenced by:
- Multiple alleles of a single gene, or
- Many genes (polygenes) working together,
- Plus, environmental factors – such as diet, light, or temperature.
All these together determine the phenotype (what we actually see).
🧬 (i) Traits Controlled by Multiple Alleles
Definition: Some genes have more than two possible alleles in a population, though each individual still carries only two. These multiple alleles can produce several phenotypes.
Example – Human ABO Blood Groups
| Allele | Produces | Type of Dominance |
|---|---|---|
| IA | Antigen A | Co-dominant |
| IB | Antigen B | Co-dominant |
| IO (i) | No antigen | Recessive |
Phenotypes:
- IAIA or IAi → Blood group A
- IBIB or IBi → Blood group B
- IAIB → Blood group AB
- ii → Blood group O
Here, three alleles (IA, IB, IO) control one gene → multiple phenotypes.
🌾 (ii) Polygenic Inheritance
Definition: Polygenic inheritance = a characteristic controlled by two or more genes, each contributing to the final phenotype.
The effects of each gene are additive or cumulative – small contributions combine to create a wide range of outcomes.
Example: Human height, skin color, grain size in wheat.
How it Works:
- Each dominant allele adds a “dose” of pigment or height.
- The more dominant alleles present → the stronger the trait.
Example:
| Number of Dominant Alleles | Phenotype |
|---|---|
| 0 | Very light skin |
| 1-2 | Light |
| 3-4 | Medium |
| 5-6 | Dark |
📈 (iii) Continuous Variation
Because many genes contribute small effects, and the environment also plays a role → the phenotypes form a gradual range rather than distinct categories. This is called continuous variation.
Examples:
- Human height
- Weight
- Skin tone
When plotted on a graph → produces a bell-shaped curve (normal distribution).
🌤️ (iv) Role of the Environment
Even with the same genotype:
- Nutrition affects height or weight.
- Sunlight affects skin color.
- Temperature affects fur color in animals (e.g., Himalayan rabbits).
Hence, phenotype = genes + environment.
🔬 Summary Table
| Feature | Multiple Alleles | Polygenic Inheritance |
|---|---|---|
| Definition | One gene, more than two alleles | Trait controlled by many genes |
| Example | ABO blood groups | Height, skin color |
| Phenotype Type | Discontinuous (distinct types) | Continuous (range of types) |
| Graph Type | Separate Bars | Bell-shaped curve |
| Environmental Effect | Minor | Major influence |
⚡ Quick Recap
Multiple alleles → 1 gene, 3+ versions (ABO system).
Polygenic inheritance → many genes add small effects.
Continuous variation → smooth gradation (e.g., height).
Environment → modifies polygenic traits further.
💡 Formula to remember:
Phenotype = Genotype (many genes) + Environment