Crossing over:-
The process of crossing over occurs by a mechanism called breakage and recunion theory of Darlington (1937). It comprises four steps-
(i) Synapsis:- The homologus chromosomes come to lie close together in pairs in the zygotene substage of the prophase of meiosis-I. Their pairing is called synapsis.
(ii) Tetrad formation:- The chromatids of each synapsed chromosome slightly separated and become visible in the pachytene substage of the prophase of meiosis. A group of four homologous chromatids is called a tetrad.
(iii) Crossing over:- Crossing over also occurs in the pachytene substage. The adjacent non sister chromatids break at homologus sites, mutually exchange small corresponding segments and rejoin.
(iv) Disjunction:- After the completion on crossing over the synaptic forces end and the homologus chromosomes move apart.
