There are two fundamentally different methods by which organisms reproduce: asexual reproduction and sexual reproduction. Asexual reproduction produces genetically identical offspring by mitotic cell division. Sexual reproduction produce offspring through the union of gametes: gametes are produced by a specialized form of cell division called meiosis. Alleles come from mutations, which can occur in two ways. Cells sometime make mistakes when copying dna prior to cell division. Sexual recombination generates genetic variability between the members of a species. Eukaryotic chromosomes usually occur in pairs with similar genetic information. Chromosomes that contain the same genes are called homologous chromosomes, or homologues. Cells with pairs of homologous chromosomes are called diploid, whi(cid:272)h (cid:373)ea(cid:374)s (cid:862)dou(cid:271)le(cid:863) A typical human cell has 23 pairs of chromosomes, for a total of 46. Twenty-two of the 23 pairs are called autosomes. The twenty-third pair are called sex chromosomes and are different in the male and female.