In the first stage of<unk> , "meiosis", the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilisation", haploid gametes come together to form a diploid zygote, and the original number of chromosomes is restored.<unk>  is a type of<unk>  that involves a complex life cycle in which a gamete (such as a sperm or egg cell) with a single set of chromosomes (haploid) combines with another to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid).<unk>  is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.<unk>  does not occur in prokaryotes (organisms without cell nuclei), but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to<unk>  in early eukaryotes. In the production of sex cells in eukaryotes, diploid mother cells divide to produce haploid cells known as gametes in a process called meiosis that involves genetic recombination. The homologous chromosomes pair up so that their DNA sequences are aligned with each other, and this is followed by exchange of genetic information between them. Two rounds of cell division then produce four haploid gametes, each with half the number of chromosomes from each parent cell, but with the genetic information in the parental chromosomes recombined. Two haploid gametes combine into one diploid cell known as a zygote in a process called fertilisation. The zygote incorporates genetic material from both gametes. Multiple cell divisions, without change of the number of chromosomes, then form a multicellular diploid phase or generation. In human<unk> , each cell contains 46 chromosomes in 23 pairs. Meiosis in the parents' gonads produces gametes that each contain only 23 chromosomes that are genetic recombinants of the DNA sequences contained in the parental chromosomes. When the nuclei of the gametes come together to form a fertilized egg or zygote, each cell of the resulting child will have 23 chromosomes from each parent, or 46 in total. In plants only, the diploid phase, known as the sporophyte, produces spores by meiosis that germinate and then divide by mitosis to form a haploid multicellular phase, the gametophyte, that produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the<unk>  haploid gametophyte and<unk>  diploid sporophyte, is known as alternation of generations. The evolution of<unk>  is considered paradoxical, because<unk>  should be able to outperform it as every young organism created can bear its own young. This implies that an<unk>  population has an intrinsic capacity to grow more rapidly with each generation. This 50% cost is a fitness disadvantage of<unk> . The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. One definite advantage of<unk>  is that it impedes the accumulation of genetic mutations.<unk>  selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for<unk> . It has been described as "a powerful evolutionary force that does not exist in<unk>  populations."
Sexual reproduction
Sex selection
Spermiogenesis
Ovarian reserve
Ectogenesis
Gender transitioning
Tumor initiation
Genital regeneration