Crossing over and chiasma formation
There are two theories to explain the relationship between crossing over and chiasma formation. These two theories will be briefly discussed in this section.

Classical theory or two plane theory. This theory was proposed by L.W. Sharp in his book Introduction to Cytology, published in 1934. According to this theory, formation of chiasmata precedes the act of genetic crossing over, so that chiasmata are not the result but the cause of crossing over. Moreover in such an interpretation, it is not necessary that there is one to one relationship between chiasmata and crossing over. The chiasmata, in this theory, represent points of accidental physical crossing of homologous, but non-sister chromatids. These chiasmata may or may not lead to breakage and subsequent exchange of chromosome segments, but whenever crossing over occurs, this results due to strain imposed by chiasma formation. In such a hypothesis, adjacent loops will have equational (sister chromatids separating) and reductional (sister chromatids not separating) separation of chromatids (Fig. 10.3 A). This theory is also known as two plane theory because it is assumed that adjacent loops would be present in different planes at right angle to each other.

Chiasmatype theory or one plane theory. Chiasmatype theory was proposed by F.A. Janssens in 1909 and was extended in 1924. It was later fully developed by J. Belling and by CD. Darlington. According to this theory, true chiasmata are the direct result of crossing over so that crossing over really precedes chiasma formation. As opposed to classical theory, in chiasmatype theory, crossing over results from breakage followed by reunion or exchange of non-sister chromatid segments from homologous chromosomes. Since in this theory, chiasmata are the result of crossing over, there would be one to one relationship between the chiasmata and crossing over. This theory is also called one plane theory as against two plane theory discussed above. In this theory, one would expect reductional separation of chromatids on either side of a chiasma (Fig. 10.3 B).