Write down the four stages involved in crossing over.

1.	Write down the four stages involved in crossing over.
Answer» 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.

Answer»

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.

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