During meiosis, pairs of homologous chromosomes often twist around each other and remain in contact during the process of separation. The point at which paired chromosomes remain in contact is known as a chiasma, because it has a cross-shaped appearance under the microscope. Breakage and rejoining of the chromosomes at these points results in crossing over, that is, the exchange of material between the chromosomes. The reciprocal genetic exchange between homologous chromosomes, which is called crossing over, changes the pattern of genes in the chromosomes. Genes that were originally linked become separated, while genes that were on different chromosomes become linked. Thus, chiasmata are associated with the separation and recombination of genetic factors. The rearrangement of genes results in differences between the chromosomes of parents and their offspring.

Cytologists first saw the points of contact between pairs of chromosomes during meiosis in the 1890s, but the descriptions were...