Edexcel A Level (IAL) Biology -3.20 Gene Interaction & Epigenetics- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.20 Gene Interaction & Epigenetics- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.20 Gene Interaction & Epigenetics- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.20 (i) understand how phenotype is the result of an interaction between genotype and the environment
(ii) know how epigenetic modification, including DNA methylation and histone modification, can alter the activation of certain genes
(iii) understand how epigenetic modifications can be passed on following cell division
Phenotype, Genotype & Epigenetic Modifications
🧬 (i) How Phenotype Results from Genotype × Environment Interaction
Key Terms:
- Genotype: Genetic makeup of an organism (the set of genes it carries).
- Phenotype: The observable characteristics (appearance, physiology, behavior).
Phenotype = Genotype + Environmental Influence
Examples:
| Example | Explanation |
|---|---|
| Plant height | Same plant species may grow taller with more sunlight and nutrients. |
| Human skin color | Genes control melanin production, but sun exposure can darken skin. |
| Hydrangea flower color | Blue in acidic soil (low pH), pink in alkaline soil – environment affects pigment. |
💡 So, even with the same genes, environmental differences can change the phenotype.
🧫 (ii) Epigenetic Modification – Turning Genes “On” or “Off”
What is Epigenetics?
“Epigenetics” means changes in gene expression without changing the DNA sequence itself.
Epigenetic marks act like switches or dimmers that control how strongly a gene is expressed.
Main Epigenetic Mechanisms:
| Type of Modification | Description | Effect on Gene Activity |
|---|---|---|
| DNA Methylation | Addition of a methyl group (-CH₃) to cytosine bases (mainly at CpG sites) | Silences the gene (prevents transcription) |
| Histone Modification | Histones = proteins DNA wraps around; they can be acetylated or deacetylated | Acetylation → gene active; Deacetylation → gene inactive |
| Non-coding RNA | Some RNA molecules regulate which genes are expressed | Can increase or decrease expression of specific genes |
🎯 Mnemonic:
“Methyl Mutes, Acetyl Activates”
→ Easy way to remember:
Methylation = Turns gene OFF
Acetylation = Turns gene ON
🌿 (iii) Passing on Epigenetic Modifications
During cell division (mitosis), many epigenetic marks are copied onto the new DNA strands.
This ensures specialized cells remember their identity (e.g., liver cells remain liver cells).
In some cases, epigenetic tags survive gamete formation and are inherited by offspring.
Example:
If a parent’s diet or stress changes DNA methylation patterns → these can sometimes affect the child’s gene activity (seen in famine studies).
📚 Biological Importance
| Function | Explanation |
|---|---|
| Cell differentiation | Different genes switched on/off in different cells |
| Response to environment | Allows flexibility – genes can be turned off or on depending on stress, diet, etc. |
| Inheritance of traits | Some acquired gene expression patterns passed to offspring |
| Disease link | Abnormal methylation patterns may lead to cancer or developmental disorders |
🧾 Quick Recap
Genotype + Environment → Phenotype: Both genes and surroundings shape traits.
Epigenetic modification: Chemical tags on DNA/histones change gene expression.
DNA methylation: Silences genes.
Histone acetylation: Activates genes.
Heritable changes: Some epigenetic tags copied during cell division.
Think of it like: DNA = script, Epigenetics = director’s notes on which scenes to act or skip!