Sexual reproduction produces greater genetic variation than asexual reproduction. Each offspring inherits a particular combination of genes from two parents; so many combinations are possible that each offspring is essentially unique, differing from its siblings and both parents. Human somatic cells (most body cells) are diploid. Each somatic cell has two sets of 23 chromosomes, making a total of 46. A special kind of cell division called meiosis occurs in the ovaries and testes, producing haploid gametes -- eggs and sperm -- that carry only one set of 23 chromosomes each. Fertilization of an egg by a sperm combines two sets of chromosomes, producing a diploid zygote with 46 chromosomes. The life cycle is completed as the zygote develops into a sexually mature diploid adult who begins producing haploid gametes. Note that all sexual life cycles involve an alternation of diploid and haploid stages. Meiosis produces haploid gametes, and fertilization produces diploid organisms.
