Chiasma

The term "Chiasma" is also sometimes used to refer to the Optic chiasm.

A chiasma (Greek χίασμα, "crossing", related to the Greek letter Χ) is the technical term for the point where two chromatids are intertwined (interwoven) in a cell. The chiasmata are thought to be the points where two nonsister 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 doesn't cause any change in the resulting daughter cells). The chiasmata become visible during the diplonema stage of meiosis, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene Stage. When tetrads, which are composed of two pairs of sister chromatids, begin 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) x 100

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