A visible change of partners or crossover in two of a group of four chromatids during the first meiotic prophase.
The point at which homologous chromosomes remain in contact after they have started to separate in the first division of meiosis. Chiasmata occur from the end of prophase to anaphase and represent the point at which mutual exchange of genetic material takes place.
This is an X-shaped crossing. The optic chiasma is where the nerve fibres from the nasal half of each retina cross over the mid line to join the optic tract from the other side.