In prophase I of meiosis, homologous chromosomes pair up and swap segments. This crossing over mixes the alleles inherited from the two parents, so every egg or sperm carries a unique combination.
Project-generated teaching illustration (AI-generated, simplified; marker positions are approximate)
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Labelled parts
Key points
Homologous chromosomes are the matching pair, one from each parent.
A tetrad contains four chromatids, and crossing over happens between non-sister chromatids.
Crossing over, plus random assortment of chromosomes, creates genetic variation.
