Crossing over is also called as recombination. It is the shift of chromosome segments between nonsister chromatids in meiosis. It generates new combinations of genes in the gametes that are not present in either parent which contribute to genetic diversity which will aid cells to compete in survival of the fittest and evolution. Genetic recombination that involves crossing over takes place during prophase I of meiosis in sex cell production. It takes place when the chiasma breaks and the defective chromosome segments get replaced onto homologous chromosomes.
Sexual reproduction results in infinite possibilities of genetic variation which results in offsprings that are genetically unique. They vary from both parents and also from each other.