Edexcel A Level (IAL) Biology -3.9 Gene Locus- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.9 Gene Locus- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.9 Gene Locus- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.9 (i) know that a locus is the location of genes on a chromosome
(ii) understand the linkage of genes on a chromosome
Gene Locus & Linkage
🌱 Introduction
Every gene has a fixed position on a chromosome, known as its locus. When genes are close together on the same chromosome, they tend to be inherited together this is called linkage.
🧩 (i) Gene Locus – Meaning
- A locus (plural loci) is the specific physical location of a gene on a chromosome.
- Each chromosome carries many genes, arranged linearly.
- The same genes occupy the same loci on homologous chromosomes.
Example:
The gene for eye color might be found at locus 15q21 on human chromosome 15.
📘 Key Idea:
If two organisms are of the same species, the order of genes (loci) on their chromosomes is usually identical.
🧬 (ii) Linkage of Genes on a Chromosome
- Linked genes are genes located on the same chromosome.
- They are inherited together, because during meiosis, the entire chromosome (not individual genes) is passed on.
- The closer two genes are on a chromosome, the less likely they will be separated by crossing over.
- If genes are far apart, crossing over may occur between them → they can be recombined and inherited separately.
🧠 Example:
In fruit flies (Drosophila):
Genes for body color and wing shape are located on the same chromosome.
If no crossing over occurs, these traits are inherited together → linkage.
If crossing over happens → new combinations of traits may appear (called recombinants).
⚖️ Summary Table
| Concept | Definition | Key Point | Example |
|---|---|---|---|
| Locus | Fixed position of a gene on a chromosome | Same genes occupy same loci on homologous chromosomes | Gene for eye color at 15q21 |
| Linked genes | Genes present on the same chromosome | Usually inherited together | Body color & wing shape in Drosophila |
| Crossing over | Exchange of chromosome segments in meiosis | Can separate linked genes | Produces recombinant offspring |
⚡ Quick Recap
Locus → Position of a gene on a chromosome.
Linkage → Genes on the same chromosome inherited together.
Closer genes = stronger linkage, less chance of separation.
Crossing over → May break linkage and form new gene combinations.
🧠 Remember: Independent assortment applies only to unlinked genes (on different chromosomes).