|
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 (2''n'', 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. Because chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote. However, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation. For example, the hearts of two-year-old children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei. Cells are described according to the number of sets present (the ploidy level): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid is frequently used to describe cells with three or more sets of chromosomes (triploid or higher ploidy). == Etymology == The term ''ploidy'' is a back-formation from ''haploid'' and ''diploid''. These two terms are from Greek ἁπλόος ''haplóos'' "single" and διπλόος ''diplóos'' "double" combined with εἶδος ''eîdos'' "form" (compare ''idol'' from Latin ''īdōlum'', that from Greek εἴδωλον ''eídōlon'' derived from εἶδος ''eîdos''). Eduard Strasburger, who coined the terms ''haploid'' and ''diploid'', based on Weismann's conception of the id (or germ plasm), used diploid to refer to an organism with twice the number of chromosomes of a haploid organism, hence "double" and "single". The two terms were borrowed from German through William Henry Lang's 1908 translation of an 1906 textbook by Strasburger and colleagues.〔Strasburger, E.; Noll, F.; Schenck, H.; Karsten, G. 1908. ''A Textbook of botany'', 3rd English ed. (1908) (), rev. with the 8th German ed. (1906) (), translation by W. H. Lang of ''Lehrbuch der Botanik für Hochschulen''. Macmillan, London.〕 Technically, ploidy is a description of a nucleus. Though at times authors may report the total ploidy of all nuclei present within the cell membrane of a syncytium,〔Encyclopedia of the Life Sciences (2002) "Polyploidy" Francesco D’Amato and Mauro Durante〕 usually the ploidy of the nuclei present will be described. For example, a fungal dikaryon with two haploid nuclei is distinguished from the diploid in which the chromosomes share a nucleus and can be shuffled together.〔(【引用サイトリンク】title=Dikaryons, diploids, and evolution )〕 Nonetheless, because in most situations there is only one nucleus, it is commonplace to speak of the ploidy of a cell. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ploidy」の詳細全文を読む スポンサード リンク
|