Ploidy is the number of sets of chromosomes in a cell. Usually a gamete (sperm or egg, which fuse into a single cell during the fertilization phase of sexual reproduction) carries a full set of chromosomes that includes a single copy of each chromosome, as aneuploidy generally leads to severe genetic disease in the offspring. The gametic or haploid number (n) is the number of chromosomes in a gamete. Two gametes form a diploid zygote with twice this number (2n, the zygotic or diploid number) i.e. two copies of autosomal chromosomes. For humans, a diploid species, n = 23. A typical human somatic cell contains 46 chromosomes: 2 complete haploid sets, which make up 23 homologous chromosome pairs.