Chiasma (genetics)

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A chiasma (plural: chiasmata), in genetics, is thought to be the point where two homologous chromatids exchange genetic material during chromosomal crossover during meiosis (sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene stage. When each bivalent, which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.

chiasma frequency = 2 x recombination frequency

where recombination frequency is:

recombination frequency = (no. of recombinants) / (total no. of progeny)