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Diploid cells within either the human testes or ovaries produce haploid sperm or eggs by undergoing two divisions during the process of meiosis.

The first division during meiosis is meiosis I, which begins with diploid cells in which chromosomes have replicated, appearing x like in shape. Critical here is the existence of such paternally and maternally inherited structures, which exchange information between them during a process called crossing over, creating genetic diversity.

Other changes during this period result in a microtubule-based framework that separates the chromosomes so that they are randomly but evenly partitioned between opposite ends of the cell, which then splits. The end result is a novel pair of haploid cells. Importantly, although every x in these newly formed structures is individually termed a chromosome, each is actually composed of two copies of the same chromosome, termed sister chromatids.

During the second division, meiosis II, a similar arrangement of microtubules breaks apart sister chromatids. This again yields haploid cells, with the key difference being that the chromosomes in them are no longer replicated and appears as I's when condensed.

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