A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is also used for the complete set of chromosomes in a species or in an individual organism[1][2][3] and for a test that detects this complement or measures the number.

Karyotypes describe the chromosome count of an organism and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.[4] The preparation and study of karyotypes is part of cytogenetics.

Karyogram of human male using Giemsa staining

The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a photomicrograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.

The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23).[2]p28 Thus, in humans 2n = 46.

So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.

The study of karyotypes is important for cell biology and genetics, and the results may be used in evolutionary biology (karyosystematics)[5] and medicine. Karyotypes can be used for many purposes; such as to study chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.

History of karyotype studies

Chromosomes were first observed in plant cells by Carl Wilhelm von 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 von Waldeyer in 1888. It is New Latin from Ancient Greek κάρυον karyon, "kernel", "seed", or "nucleus", and τύπος typos, "general form").

The next stage took place after the development of genetics in the early 20th century, when it was appreciated that chromosomes (that can be observed by karyotype) were the carrier of genes. Lev Delaunay [ru] in 1922 seems to have been the first person to define the karyotype as the phenotypic appearance of the somatic chromosomes, in contrast to their genic contents.[6][7] The subsequent history of the concept can be followed in the works of C. D. Darlington[8] and Michael JD White.[2][9]

Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain?[10] In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[11] Painter in 1922 was not certain whether the diploid of humans was 46 or 48, at first favoring 46,[12] but revised his opinion from 46 to 48, and he correctly insisted on humans having an XX/XY system.[13] Considering the techniques of the time, these results were remarkable.

Fusion of ancestral chromosomes left distinctive remnants of telomeres, and a vestigial centromere

In textbooks, 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[14] was responsible for finding the approach:

  1. Using cells in tissue culture
  2. Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes
  3. Arresting mitosis in metaphase by a solution of colchicine
  4. Squashing the preparation on the slide forcing the chromosomes into a single plane
  5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram.

The work took place in 1955, and was published in 1956. The karyotype of humans includes only 46 chromosomes.[15][16] The great apes have 48 chromosomes. Human chromosome 2 is now known to be a result of an end-to-end fusion of two ancestral ape chromosomes.[17][18]

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Simple English: Karyotype
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