Edexcel iGCSE Biology-4.30 & 4.31 Random Fertilisation & Genetic Variation- Study Notes- New Syllabus
Edexcel iGCSE Biology-4.30 & 4.31 Random Fertilisation & Genetic Variation- Study Notes- New syllabus
Edexcel iGCSE Biology-4.30 & 4.31 Random Fertilisation & Genetic Variation- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
4.30 understand how division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes
4.31 understand how random fertilisation produces genetic variation of offspring
Meiosis – Formation of Genetically Different Haploid Gametes
🔹 Introduction
Meiosis = a special type of cell division in diploid cells.
Produces four haploid cells (gametes), each with half the number of chromosomes.
Key purpose → ensures genetic variation in offspring.
📌 Key Features of Meiosis
- Occurs in sex organs: ovaries (female) & testes (male).
- Starts with one diploid cell (2n) → ends with four haploid cells (n).
- Two divisions:
- Meiosis I → homologous chromosomes separate → reduces chromosome number by half.
- Meiosis II → sister chromatids separate → 4 genetically different cells.
- Each gamete is genetically unique because of:
- Independent assortment of chromosomes
- Crossing over (exchange of DNA between homologous chromosomes)
🧬 Steps of Meiosis
Meiosis I – Reduction Division
Homologous chromosomes pair up → exchange segments (crossing over) → separate.
Result: 2 cells, each haploid, but chromosomes still duplicated.
Meiosis II – Equational Division
Sister chromatids separate → each of the 2 cells divides again.
Result: 4 haploid gametes, all genetically different.
📊 Summary Table
| Feature | Meiosis | Outcome |
|---|---|---|
| Chromosome number | Diploid → Haploid | 4 gametes (n) |
| Genetic similarity | Unique, not identical | Genetic variation in offspring |
| Number of divisions | 2 (Meiosis I & II) | Reduction + equational |
| Location | Sex organs | Gametes: sperm & egg |
| Special events | Crossing over, independent assortment | Ensures diversity |
📝 Quick Recap
Meiosis → diploid → 4 haploid genetically different gametes.
Importance → introduces genetic variation in offspring.
Key mechanisms:
– Crossing over → exchange DNA
– Independent assortment → random combination of chromosomes
Gametes fuse at fertilisation → restores diploid number in zygote.
Random Fertilisation – Creating Genetic Variation
🔹 Introduction
Fertilisation = fusion of male gamete (sperm) and female gamete (egg).
When gametes fuse randomly, it produces offspring with unique combinations of genes.
This adds genetic variation, which is essential for evolution and survival.
📌 Key Points
- Gametes are haploid (n) → combine to form diploid zygote (2n).
- Each sperm and egg carries different genetic info (from meiosis).
- Random fertilisation → huge number of possible genetic combinations.
🧬 Example
Human: 23 chromosomes in sperm × 23 chromosomes in egg → 2²³ combinations from each parent.
Total possible combinations in zygote = 2²³ × 2²³ ≈ 70 trillion!
This explains why siblings (except identical twins) are genetically unique.
📊 Summary Table
| Feature | Explanation | Example |
|---|---|---|
| Haploid gametes | Carry half chromosomes | Sperm (n=23), Egg (n=23) |
| Random fertilisation | Any sperm can fuse with any egg | Offspring are unique |
| Genetic variation | Differences in DNA of offspring | Siblings differ genetically |
| Importance | Helps species adapt & survive | Evolutionary advantage |
📝 Quick Recap
Random fertilisation → any sperm + any egg → unique zygote.
Works on top of meiosis, which already produces diverse gametes.
Ensures offspring are genetically different → survival & evolution.
Huge combinations possible (e.g., ~70 trillion in humans).