Q.What is crossing over ?

Crossing over exchanges segments between homologous chromatids during meiosis, producing new allele combinations.

During prophase I of meiosis, homologous chromosomes first pair closely together (synapsis) at zygotene to form bivalents (tetrads), held together by the synaptonemal complex. At the pachytene stage, non-sister chromatids of the paired homologues break at corresponding points and exchange the broken segments, which then rejoin -- this reciprocal exchange of chromosome segments is called crossing over. The points where crossing over has occurred become visible at diplotene as X-shaped structures called chiasmata, which also help hold the homologous chromosomes together until anaphase I. Because crossing over shuffles alleles between maternal and paternal chromatids, it is the principal source of genetic recombination and is responsible for the variation seen among offspring beyond that produced by independent …