Name: Incomplete Linkage in Drosophila
Uploaded: 2019-07-19
Duration: 8 min 39 s
Description: Completely linked genes show zero recombination. What about incomplete linkage? How can we predict the exact number of recombinants in a progeny? Learn about...

Question

Explain the mechanism of crossing over.

Accepted Answer

Crossing over takes place during meiotic prophase, i.e., during pachytene. It involves following steps :

1. Synapsis: Crossing over occurs between non-sister chromatids. Thus, one chromatid from each of the two homologus chromosomes is involved in crossing over. The pairing phenomenon of homologous chromosome is called synapsis.

2. Tetrad formation: Each crossing over involves only two of the four chromatids of two homologus chromosomes. At this stage each bivalent has four chromatids. This stage is called tetrad stage.

3. Cross Over: The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called chiasma (pleural chiasmata). It is thought to be the place where crossing over takes place. Depending on the position, chiasma is of two types, terminal and interstitial.

When the chiasma is located at the end of the pairing chromatids, it is known as terminal chiasma and when it is located in the middle part of non-sister chromatids, it is referred to as interstitial chiasma.

4. Terminalization: The movement of chiasma away from the centromere and towards the end of tetrads is called terminalization. The chiasma terminalization occurs between diplotene and metaphase I.

Explain the mechanism of crossing over.

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