Editing Cytogenetics (section)

==History==

===Beginnings===

Chromosomes were first observed in plant cells by [[Carl Nägeli]] in 1842. Their behavior in animal ([[salamander]]) cells was described by [[Walther Flemming]], the discoverer of [[mitosis]], in 1882. The name was coined by another German anatomist, [[Heinrich Wilhelm Gottfried von Waldeyer-Hartz|von Waldeyer]] in 1888.

The next stage took place after the development of genetics in the early 20th century, when it was appreciated that the set of chromosomes (the [[karyotype]]) was the carrier of the genes. Levitsky seems to have been the first to define the karyotype as the [[phenotypic]] appearance of the [[Somatic (biology)|somatic]] chromosomes, in contrast to their [[gene|genic]] contents.<ref>{{cite book |last1=Levitsky |first1=Grigorii Andreevich |title=Material'nye osnovy nasledstvennosti |trans-title=The Material Basis of Heredity |language=ru |location=Kiev |publisher=Gosizdat Ukrainy |year=1924 }}{{page needed|date=October 2020}}</ref><ref>{{cite journal |author=Levitsky GA |year=1931 |title=The morphology of chromosomes |journal=Bull. Applied Bot. Genet. Plant Breed |volume=27 |pages=19–174}}</ref> Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal [[diploid]] human cell contain?<ref>{{cite journal |last1=Kottler |first1=Malcolm Jay |title=From 48 to 46: cytological technique, preconception, and the counting of human chromosomes. |journal=Bulletin of the History of Medicine |date=1974 |volume=48 |issue=4 |pages=465–502 |id={{ProQuest|1296285397}} |jstor=44450164 |pmid=4618149 }}</ref> 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 |vauthors=von Winiwarter H |year=1912 |title=Études sur la spermatogenese humaine |trans-title=Human spermatogenesis studies |language=fr |journal=Arch. Biologie |volume=27 |issue=93 |pages=147–149 }}</ref> [[Theophilus Painter|Painter]] in 1922 was not certain whether the diploid number of humans was 46 or 48, at first favoring 46.<ref>Painter T.S. "The spermatogenesis of man" p. 129 in {{cite journal |title=Abstracts |journal=The Anatomical Record |date=January 1922 |volume=23 |issue=1 |pages=89–132 |doi=10.1002/ar.1090230111 |doi-access=free }}</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 of sex-determination.<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> Considering their techniques, these results were quite remarkable. In science books, the number of human chromosomes remained at 48 for over thirty years. New techniques were needed to correct this error. [[Joe Hin Tjio]] working in [[Albert Levan]]'s lab<ref>{{cite news|url=https://www.theguardian.com/news/2001/dec/11/guardianobituaries.medicalscience|title=Joe Hin Tjio The man who cracked the chromosome count|work=[[The Guardian]]|author=Wright, Pearce|date=11 December 2001|url-status=live|archive-url=https://web.archive.org/web/20170825151411/https://www.theguardian.com/news/2001/dec/11/guardianobituaries.medicalscience|archive-date=25 August 2017}}</ref><ref>{{cite news|url=https://www.nytimes.com/2001/12/07/us/joe-hin-tjio-82-research-biologist-counted-chromosomes.html|title=Joe Hin Tjio, 82; Research Biologist Counted Chromosomes|work=[[The New York Times]]|author=Saxon, Wolfgang|date=7 December 2001|url-status=live|archive-url=https://web.archive.org/web/20130512193836/http://www.nytimes.com/2001/12/07/us/joe-hin-tjio-82-research-biologist-counted-chromosomes.html|archive-date=12 May 2013}}</ref> was responsible for finding the approach:

:# Using cells in culture
:# Pre-treating cells in a [[Hypotonicity|hypotonic solution]], which swells them and spreads the chromosomes
:# Arresting [[mitosis]] in [[metaphase]] by a solution of [[colchicine]]
:# 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 1956 for it to be generally accepted that the karyotype of man included only 46 chromosomes.<ref>{{cite journal |last1=Tjio |first1=Joe Hin |last2=Levan |first2=Albert |title=The chromosome number of man |journal=Hereditas |date=9 July 2010 |volume=42 |issue=1–2 |pages=723–4 |doi=10.1111/j.1601-5223.1956.tb03010.x |pmid=345813 |doi-access=free }}</ref><ref>{{cite book |last1=Hsu |first1=T. C. |title=Human and Mammalian Cytogenetics: An Historical Perspective |date=2012 |publisher=Springer Science & Business Media |isbn=978-1-4612-6159-9 }}{{page needed|date=October 2020}}</ref><ref>{{cite web |url=http://www.britannica.com/EBchecked/topic/228983/human-genetics/50731/The-human-chromosomes |title=Human genetics (Biology) :: The human chromosomes -- Britannica Online Encyclopedia |access-date=2011-03-15 |url-status=live |archive-url=https://web.archive.org/web/20110217045905/http://www.britannica.com/EBchecked/topic/228983/human-genetics/50731/The-human-chromosomes |archive-date=2011-02-17 }} Encyclopædia Britannica, The Human Chromosome</ref> The [[great apes]] have 48 chromosomes. [[Human chromosome 2]] was formed by a merger of ancestral chromosomes, reducing the number.<ref>{{cite web |url=http://www.evolutionpages.com/chromosome_2.htm |title=Chromosome fusion |access-date=2010-05-29 |url-status=live |archive-url=https://web.archive.org/web/20110809040210/http://www.evolutionpages.com/chromosome_2.htm |archive-date=2011-08-09 }} Evolution Pages, Chromosome fusion</ref>