Editing Chromosome (section)

== Karyotype ==
{{Main|Karyotype}}
[[File:NHGRI human male karyotype.png|thumb|upright=0.9|Karyogram of a human male]]
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyogram]] of a human, with annotated [[Locus (genetics)|bands and sub-bands]]. It is a graphical representation of the idealized human [[diploid]] karyotype. It shows dark and white regions on [[G banding]]. Each row is vertically aligned at [[centromere]] level. It shows 22 [[homologous chromosome]]s, both the female (XX) and male (XY) versions of the [[sex chromosome]] (bottom right), as well as the [[Human mitochondrial genetics|mitochondrial genome]] (at bottom left). {{further|Karyotype}}]]

In general, the [[karyotype]] is the characteristic chromosome complement of a [[eukaryote]] [[species]].<ref>{{cite book |author=White, M. J. D. |title=The chromosomes |url=https://archive.org/details/chromosomes01whit |url-access=registration |publisher=Chapman and Hall, distributed by Halsted Press, New York |location=London |year=1973 |page=28 |isbn=978-0-412-11930-9 |edition=6th}}</ref> The preparation and study of karyotypes is part of [[cytogenetics]].

Although the [[DNA replication|replication]] and [[Transcription (genetics)|transcription]] of [[DNA]] is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization.
In some cases, there is significant variation within species. Often there is:
:1. variation between the two sexes
:2. variation between the [[germline]] and [[Somatic cell|soma]] (between [[gamete]]s and the rest of the body)
:3. variation between members of a population, due to [[Polymorphism (biology)|balanced genetic polymorphism]]
:4. [[Allopatric speciation|geographical variation]] between [[Race (classification of human beings)|races]]
:5. [[Mosaic (genetics)|mosaics]] or otherwise abnormal individuals.
Also, variation in karyotype may occur during development from the fertilized egg.

The technique of determining the karyotype is usually called ''karyotyping''. Cells can be locked part-way through division (in metaphase) [[in vitro]] (in a reaction vial) with [[colchicine]]. These cells are then stained, photographed, and arranged into a ''karyogram'', with the set of chromosomes arranged, autosomes in order of length, and sex chromosomes (here X/Y) at the end.

Like many sexually reproducing species, humans have special [[XY sex-determination system|gonosomes]] (sex chromosomes, in contrast to [[autosome]]s). These are XX in females and XY in males. <!--- Irrelevant in this section:"In females, one of the two X chromosomes is inactive and can be seen under a microscope as [[Barr body|Barr bodies]]."--->

=== History and analysis techniques ===
{{See also|Argument from authority#Use in science}}
Investigation into the human karyotype took many years to settle the most basic question: ''How many chromosomes does a normal [[diploid]] human cell contain?'' In 1912, [[Hans von Winiwarter]] reported 47 chromosomes in [[spermatogonia]] and 48 in [[oogonia]], concluding an [[XO sex-determination system|XX/XO]] [[Sex-determination system|sex determination mechanism]].<ref>{{cite journal |author=von Winiwarter H |title=Études sur la spermatogenèse humaine |journal=Archives de Biologie |volume=27 |issue=93 |pages=147–9 |year=1912}}</ref> In 1922, [[Theophilus Painter|Painter]] was not certain whether the diploid number of man is 46 or 48, at first favouring 46.<ref>{{cite journal |author=Painter TS |title=The spermatogenesis of man |journal=Anat. Res.|volume=23 |page=129 |year=1922}}</ref> He revised his opinion later from 46 to 48, and he correctly insisted on humans having an [[XY sex-determination system|XX/XY]] system.<ref>{{cite journal|last1=Painter|first1=Theophilus S.|title=Studies in mammalian spermatogenesis. II. The spermatogenesis of man|journal=Journal of Experimental Zoology|date=April 1923|volume=37|issue=3|pages=291–336|doi=10.1002/jez.1400370303|bibcode=1923JEZ....37..291P }}</ref>

New techniques were needed to definitively solve the problem:
# Using cells in culture
# Arresting [[mitosis]] in [[metaphase]] by a solution of [[colchicine]]
# Pretreating cells in a [[Hypotonicity|hypotonic solution]] {{nowrap|0.075 M KCl}}, which swells them and spreads the chromosomes
# Squashing the preparation on the slide forcing the chromosomes into a single plane
# Cutting up a photomicrograph and arranging the result into an indisputable karyogram.

It took until 1954 before the human diploid number was confirmed as 46.<ref>{{cite journal |doi=10.1111/j.1601-5223.1956.tb03010.x | vauthors = Tjio JH, Levan A | title=The chromosome number of man |journal=Hereditas |volume=42 |pages=723–4 |year=1956 |issue=1–2| pmid = 345813 |hdl=10261/15776 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Ford CE, Hamerton JL | title = The chromosomes of man | journal = Nature | volume = 178 | issue = 4541 | pages = 1020–3 | date = November 1956 | pmid = 13378517 | doi = 10.1038/1781020a0 | bibcode = 1956Natur.178.1020F | s2cid = 4155320 }}</ref> Considering the techniques of Winiwarter and Painter, their results were quite remarkable.<ref>Hsu T.C. (1979) ''Human and mammalian cytogenetics: a historical perspective''. Springer-Verlag, N.Y. {{ISBN|9780387903644}} p. 10: "It's amazing that he [Painter] even came close!"</ref> [[Pan (genus)|Chimpanzees]], the closest living relatives to modern humans, have 48 chromosomes as do the other [[great apes]]: in humans two chromosomes fused to form [[chromosome 2]].