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4.3.5 Meiosis: Sources of Genetic Variation

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Meiosis produces genetically different cells through two key processes that introduce variation: independent assortment of chromosomes and crossing over of chromatids. These mechanisms ensure that offspring inherit unique combinations of genetic material.

Sources of Genetic Variation

Independent Assortment of Chromosomes

During meiosis I, homologous chromosomes line up in pairs along the equator of the cell.
The orientation of each pair is random, meaning that the maternal or paternal chromosome can go to either daughter cell.
This creates different combinations of chromosomes in the gametes.
For example, in humans with 23 pairs of chromosomes, there are 2²³ (over 8 million) possible combinations of chromosomes from independent assortment alone.

Crossing Over of Chromatids

Crossing over occurs during prophase I of meiosis when homologous chromosomes form pairs called bivalents.
The chromatids of homologous chromosomes intertwine and sections of DNA are exchanged between them at points called chiasmata.
This leads to a new combination of alleles on each chromosome, increasing genetic diversity.
For example, a chromosome may now carry alleles from both the maternal and paternal chromosomes.

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Summary

Independent assortment results in random combinations of whole chromosomes in gametes.
Crossing over creates recombinant chromosomes with unique combinations of alleles.
These processes ensure that each gamete is genetically unique, contributing to variation in offspring when gametes fuse during fertilisation.

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Key Exam Tips

Use diagrams to illustrate independent assortment and crossing over in meiosis I.
Remember: crossing over happens during prophase I, and independent assortment occurs during metaphase I.
Understand how these mechanisms contribute to evolution by increasing genetic variation in populations.

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