Editing Karyotype (section)

== Observations on karyotypes ==
[[File:Condensation and resolution of human sister chromatids in early mitosis.svg|thumb|Chromosomes at various stages of [[mitosis]]. Karyograms are generally made by chromosomes in prometaphase or metaphase. During these phases, the two copies of each chromosome (connected at the [[centromere]]) will look as one unless the image resolution is high enough to distinguish the two.]]
[[File:Human Chromosomes (crop).jpg|thumb|Micrograph of human chromosomes before further processing. Staining with Giemsa confers a purple color to chromosomes, but micrographs are often converted to [[grayscale]] to facilitate data presentation and make comparisons of results from different laboratories.<ref>{{cite book|url=http://www.informatics.jax.org/silver/chapters/5-2.shtml|title=Mouse Genetics, Concepts and Applications. Chapter 5.2: KARYOTYPES, CHROMOSOMES, AND TRANSLOCATIONS|author=Lee M. Silver|year=1995|publisher=Oxford University Press}} Revised August 2004, January 2008</ref>]]

=== Staining ===

The study of karyotypes is made possible by [[staining]]. Usually, a suitable [[dye]], such as [[Giemsa]],<ref>A preparation which includes the dyes Methylene Blue, Eosin Y and Azure-A,B,C</ref> is applied after [[Cell (biology)|cells]] have been arrested during [[cell division]] by a solution of [[colchicine]] usually in [[metaphase]] or [[prometaphase]] when most condensed. In order for the [[Giemsa]] stain to adhere correctly, all chromosomal proteins must be digested and removed. For humans,  [[white blood cells]] are used most frequently because they are easily induced to divide and grow in [[tissue culture]].<ref name="Gustashaw K.M 1991">Gustashaw K.M. 1991. Chromosome stains. In ''The ACT Cytogenetics Laboratory Manual'' 2nd ed, ed. M.J. Barch. The Association of Cytogenetic Technologists, Raven Press, New York.</ref> Sometimes observations may be made on non-dividing ([[interphase]]) cells. The sex of an unborn [[fetus]] can be predicted by observation of interphase cells (see [[Amniocentesis|amniotic centesis]] and [[Barr body]]).

=== Observations ===

Six different characteristics of karyotypes are usually observed and compared:<ref>{{cite book |last=Stebbins |first=G.L. |title=Chromosomal evolution in higher plants |url=https://archive.org/details/chromosomalevolu0000steb |url-access=registration |publisher=Arnold |location=London |year=1971 |pages=[https://archive.org/details/chromosomalevolu0000steb/page/85 85–86]|isbn=9780713122879 }}</ref>

# Differences in absolute sizes of chromosomes.  Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family.  For example, the legumes ''[[Lotus tenuis]]'' and ''[[Vicia faba]]'' each have six pairs of chromosomes, yet ''V. faba'' chromosomes are many times larger.  These differences probably reflect different amounts of DNA duplication.
# Differences in the position of [[centromeres]].  These differences probably came about through [[translocations]].
# Differences in relative size of chromosomes.  These differences probably arose from segmental interchange of unequal lengths.
# Differences in basic number of chromosomes.  These differences could have resulted from successive unequal translocations which removed all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis) or through fusion.  Humans have one pair fewer chromosomes than the great apes.  Human chromosome 2 appears to have resulted from the fusion of two ancestral chromosomes, and many of the genes of those two original chromosomes have been translocated to other chromosomes.
# Differences in number and position of satellites.  [[Satellite chromosome|Satellites]] are small bodies attached to a chromosome by a thin thread.
# Differences in degree and distribution of [[GC content]] ([[Guanine]]-[[Cytosine]] pairs versus [[Adenine]]-[[Thymine]]). In metaphase where the karyotype is typically studied, all DNA is condensed, but most of the time, DNA with a high GC content is usually less condensed, that is, it tends to appear as [[euchromatin]] rather than [[heterochromatin]]. GC rich DNA tends to contain more [[coding DNA]] and be more [[Transcription (DNA)|transcriptionally active]].<ref name=Romiguier2017/> GC rich DNA is lighter on [[Giemsa staining]].<ref name="ReferenceA">Thompson & Thompson Genetics in Medicine 7th Ed</ref> Euchromatin regions contain larger amounts of [[Guanine]]-[[Cytosine]] pairs (that is, it has a higher [[GC content]]). The staining technique using [[Giemsa]] staining is called [[G banding]] and therefore produces the typical "G-Bands".<ref name="ReferenceA"/>

A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.

Variation is often found:

# between the sexes,
# between the [[germ-line]] and [[somatic (biology)|soma]] (between [[gametes]] and the rest of the body),
# between members of a population ([[polymorphism (biology)|chromosome polymorphism]]),
# in [[allopatric speciation|geographic specialization]], and
# in [[Mosaic (genetics)|mosaics]] or otherwise abnormal individuals.<ref name="White1"/>