Chromosomal crossover occurs when two chromosomes exchange portions of genetic material. Chromosomes are formed of DNA and contain a great number of genes. When meiosis occurs, allowing the replication of DNA in a cell, the chromosomes "cross over" each other, attaching at a point and then breaking off with a portion of the other chromosome's genes. This allows the genes to become a healthy, unique mix of genes and material instead of simply replicating a clone; chromosomal crossover is vital in embryo development and growing cells.
Parent cells, which contribute their DNA to a daughter cell, usually carry two alleles, or copies of each chromosome. Each resulting daughter chromosome, and then daughter cell, thus has a unique genetic makeup, and can then contribute its own alleles to new cells; this allows greater genetic diversity in new cellular structure and new organisms, and lowers the risks from a small gene pool.