Q.What is crossing over ? Give its significance.

Crossing over swaps chromosome segments between homologous chromatids, creating new gene combinations and enabling genetic mapping.

Crossing over is the reciprocal exchange of genetic material between non-sister chromatids of a pair of homologous chromosomes, occurring at the pachytene stage of prophase I of meiosis, mediated by the synaptonemal complex and enzyme complexes at recombination nodules; cytologically it is visualised as chiasmata at the diplotene stage.

Significance:

1. Source of genetic recombination/variation -- it generates new combinations of alleles on a chromosome that were not present in either parent, increasing genetic diversity within a population -- the raw material on which natural selection and evolution act.
2. Basis of genetic (linkage) mapping -- since the frequency of recombination between two genes is roughly proportional to the physical distance separating them on the chromosome, recombination frequencies are used to construct genetic maps (in map units/centimorgans). …