Sexual reproduction is the process whereby two parents produce offspring genetically different from themselves that have new combinations of the parents' characteristics. This contrasts with asexual reproduction, where one parent produces offspring genetically identical to itself. During sexual reproduction, each parent contributes one haploid gamete (a sex cell with half the normal number of chromosomes). The two sex cells fuse during fertilization and form a diploid zygote (which has the normal number of chromosomes). Recombination, which is the production of variations in gene combinations, occurs at fertilization, bringing together new combinations of alleles. Crossing over, the exchange of pieces of chromosomes by two homologous chromosomes, also brings about genetic variation during sexual reproduction. Sexual reproduction is advantageous because it generates variations in characters that can adapt a species over time and improve its chances of survival.

Sexual reproduction occurs in practically all forms of life. Even...