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In humans, male and female germ cells undergo crossing over or genetic recombination to ensure a distinct offspring.

Crossing over occurs during the prophase I of meiosis I between homologous chromosome pairs inherited from each parent.

Before meiosis I starts, the loosely arranged chromatin inside the nucleus duplicates itself during the S phase of the cell cycle. As the cells enter prophase I, the duplicated chromatin condenses to form sets of X-shaped chromosomes.

Each arm of an X is a copy of the same parental chromosome called sister chromatids. In contrast, chromatids of a homologous chromosome pair are called non-sister chromatids.

As the homologous chromosomes pair up, a protein framework called the synaptonemal complex  links them into bivalents.

As the same genes of the bivalents align, they begin to intertwine. The intersecting segment breaks off and reattaches to opposite chromosomes, resulting in genetic recombination. These points of genetic transfer between the homologous pairs are called chiasma.

As the synaptonemal complex disassembles, the chiasma, together with cohesins that tightly hold the sister chromatids, keep the homologs attached until they are correctly segregated into two daughter cells.