Meiosis and Genetic Variation
Meiosis
Meiosis is the type of cell division that produces gametes (sex cells). Unlike mitosis, it produces four genetically different, haploid daughter cells from one diploid parent cell. Haploid means half the chromosome number — so when two gametes fuse at fertilisation, the diploid number is restored.
Two divisions
Meiosis involves two divisions:
- Meiosis I — homologous chromosomes are separated (reducing the chromosome number from diploid to haploid). This is the reduction division.
- Meiosis II — sister chromatids are separated (similar to mitosis).
One diploid cell → four haploid cells.
How meiosis creates genetic variation
Two key processes shuffle the genetic material, so gametes (and offspring) are all different:
1. Crossing over (in Meiosis I)
Homologous chromosomes pair up and swap sections of chromatid at points called chiasmata. This produces new combinations of alleles on the chromosomes (recombination).
2. Independent segregation (assortment)
Homologous pairs line up randomly at the equator, so which chromosome of each pair goes into which gamete is random. With 23 pairs in humans this alone gives 2²³ (~8 million) combinations.
Together with random fertilisation (any sperm can fertilise any egg), these produce enormous genetic variation — the raw material for natural selection.
Mitosis vs meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Chromosome number | Same (diploid) | Halved (haploid) |
| Genetically | Identical | Different (variation) |
| Purpose | Growth, repair | Producing gametes |
Chromosome mutations
Errors in meiosis can change chromosome number — non-disjunction (chromosomes fail to separate) can lead to conditions such as Down's syndrome (an extra chromosome 21).
Worked example
Give two ways meiosis increases genetic variation.
- Crossing over (homologous chromosomes swap alleles at chiasmata) and independent segregation (random arrangement of homologous pairs at the equator). ✓
Common mistakes
- Saying meiosis produces two cells — it produces four (two divisions).
- Confusing crossing over (swapping sections) with independent segregation (random arrangement).
- Forgetting the daughter cells are haploid and genetically different.
Exam tips
- Contrast mitosis and meiosis clearly (divisions, number, ploidy, variation, purpose).
- Explain the two sources of variation in meiosis, plus random fertilisation.
- Link haploid gametes → diploid restored at fertilisation.
Key facts to remember
- Meiosis = two divisions → four haploid, genetically different gametes; restores diploid number at fertilisation.
- Variation from crossing over (Meiosis I, at chiasmata) and independent segregation (random assortment), plus random fertilisation.
- Contrast with mitosis: mitosis = 2 identical diploid cells for growth/repair.