Meiosis and Genetic Variation

While mitosis produces genetically identical cells, meiosis is the type of cell division that produces genetically unique cells with half the chromosome number. Meiosis is essential for sexual reproduction and is a major source of genetic variation. The Edexcel A-Level Biology specification (9BI0) requires you to understand the process of meiosis and how it generates genetic diversity.

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Why Meiosis Is Necessary

In sexually reproducing organisms, body cells are diploid (2n) — they contain two complete sets of chromosomes (one from each parent). The human diploid number is 46 chromosomes (23 pairs).

If gametes (sex cells) were produced by mitosis, they would also be diploid. When two diploid gametes fused at fertilisation, the offspring would have 4n chromosomes, and this number would double with every generation. This would be unsustainable.

Meiosis solves this problem by producing haploid (n) gametes, each containing half the number of chromosomes of the parent cell. In humans, this means each gamete contains 23 chromosomes. When two haploid gametes fuse at fertilisation, the diploid number is restored.

$\text{Diploid cell (2n)} \xrightarrow{\text{meiosis}} \text{Four haploid cells (n)}$Diploid cell (2n)meiosis​Four haploid cells (n)

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Homologous Chromosomes

Homologous chromosomes (homologues) are pairs of chromosomes that: