section_path: Sexual reproduction
section_name: Sexual reproduction
paragraph_text: Sexual reproduction
  
  Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to eukaryotes. Genetic traits are encoded in the deoxyribonucleic acid (DNA) of chromosomes. The eukaryote cell has a set of paired homologous chromosomes, one from each parent, and this double-chromosome stage is called " diploid ". During sexual reproduction, a diploid organism produces specialized haploid sex cells called gametes via meiosis, each of which has a single set of chromosomes. Meiosis involves a stage of genetic recombination via chromosomal crossover, in which regions of DNA are exchanged between matched pairs of chromosomes, to form new chromosomes, each with a new combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. When gametes fuse during fertilization, the resulting zygote has half of the genetic material of the mother and half of the father. The combination of chromosomal crossover and fertilization, bringing the two single sets of chromosomes together to make a new diploid zygote, results in a new organism that contains a different set of the genetic traits of each parent.
  
  In animals, the haploid stage only occurs in the gametes, the sex cells that fuse to form a zygote that develops directly into a new diploid organism. In a plant species, the diploid organism produces a type of haploid spore by meiosis that is capable of undergoing repeated cell division to produce a multicellular haploid organism. In either case, the gametes may be externally similar (isogamy) as in the green alga Ulva or may be different in size and other aspects (anisogamy). The size difference is greatest in oogamy, a type of anisogamy in which a small, motile gamete combines with a much larger, non-motile gamete.
  
  In anisogamic organisms, by convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces large gametes is female, and one that produces small gametes is male. An individual that produces both types of gamete is a hermaphrodite. In some species, a hermaphrodite can self-fertilize and produce an offspring on its own.