A karyotype is the characteristic chromosomecomplement of a eukaryotespecies.[1][2] The preparation and study of karyotypes is part of cytogenetics.
The basic number of chromosomes in the somaticcells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n=46. In the germ-line(the sex cells) the chromosome number is n (humans: n=23).[1]
So, in normal diploidorganisms, autosomalchromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploidcells have multiple copies of chromosomes and haploidcells have single copies. The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) 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 study of karyotypes is made possible by staining. Usually, a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine.[3] Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn fetuscan be determined by observation of interphase cells (see amniotic centesisand Barr body).
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for womencontain two X chromosomesand are denoted 46,XX; menhave both an X and a Y chromosomedenoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Karyotypes can be used for many purposes; such as, to study chromosomal aberrations, cellularfunction, taxonomicrelationships, and to gather information about past evolutionaryevents.
Six different characteristics of karyotypes are usually observed and compared:[4]
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
Mixed up pieces
Cri-du-chat Syndrome: "Cry of the Cat" Syndrome; This is a terminal deletion: a part of the short arm near the end of chromosome 5 is deleted. The name of this deletion refers to cry of children who have this defect; they suffer from mental retardation, a shortened life span, and a distinctive facial appearance. It occurs one in 50,000 births.
Learn more about another important tool used in genetic analysis: pedigrees!
Levitsky seems to have been the first to define the karyotype as the phenotypicappearance of the somaticchromosomes, in contrast to their geniccontents.[6][7] The subsequent history of the concept can be followed in the works of Darlington[8] and White.[9][10]
Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploidhuman cell contain?[11] In 1912, Hans von Winiwarterreported 47 chromosomes in spermatogoniaand 48 in oogonia, concluding an XX/XOsex determinationmechanism.[12] Painterin 1922 was not certain whether the diploid number of man was 46 or 48, at first favouring 46.[13] He revised his opinion later from 46 to 48, and he correctly insisted on man having an XX/XYsystem.[14] Considering their techniques, these results were quite remarkable.
It took until the mid 1950s until it became generally accepted that the karyotype of man included only 46 chromosomes.[15][16] Rather interestingly, the great apeshave 48 chromosomes. Human chromosome 2was formed by a merger of ancestral chromosomes, reducing the number.
Although the replicationand transcriptionof DNAis highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable between species in chromosome number and in detailed organization despite being constructed out of the same macromolecules. This variation provides the basis for a range of studies in what might be called evolutionary cytology.
In some cases there is even significant variation within species. In a review, Godfrey and Masters (2000) conclude: "In our view, it is unlikely that one process or the other can independently account for the wide range of karyotype structures that are observed... But used in conjunction with other phylogenetic data, karyotypic fissioning may help to explain dramatic differences in diploid numbers between closely related species, which were previously inexplicable.[17].
Instead of the usual gene repression, some organisms go in for large-scale elimination of heterochromatin, or other kinds of visible adjustment to the karyotype.
A spectacular example of variability between closely related species is the muntjac, which was investigated by Kurt Benirschkeand his colleague Doris Wurster. The diploid number of the Chinese muntjac, Muntiacus reevesi, was found to be 46, all telocentric. When they looked at the karyotype of the closely related Indian muntjac, Muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes.[22]
The number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. The low record is held by the nematodeParascaris univalens, where the haploidn = 1; the high record would be somewhere amongst the ferns, with the Adder's Tongue Fern Ophioglossum ahead with an average of 1262 chromosomes.[24] Top score for animals might be the shortnose sturgeonAcipenser brevirostrum at a mere 372 chromosomes.[25] The existence of supernumerary or B chromosomesmeans that chromosome number can vary even within one interbreeding population; and aneuploidsare another example, though in this case they would not be regarded as normal members of the population.
The detailed study of chromosome bandingin insects with polytene chromosomescan reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilidsby Hampton Carson.
In about 6,500 square miles, the Hawaiian islands have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows. These roughly 800 Hawaiian drosophilid species are usually assigned to two genera Drosophila and Scaptomyza in the family Drosophilidae.
The polytene banding of the 'picture wing' group, the best-studied group of Hawaiian drosophilids, enabled Carson to work out the evolutionary tree long before genome analysis was practicable. In a sense, gene arrangements are visible in the banding patterns of each chromosome. Chromosome rearrangements, especially inversions, make it possible to see which species are closely related.
The results are clear. The inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. Using K-Ardating, the present islands date from 0.4 million years ago (mya) (Mauna Kea) to 10mya (Necker). The oldest member of the Hawaiian archipelagostill above the sea is Kure Atoll, which can be dated to 30 mya. The archipelago itself (produced by the Pacific platemoving over a hot spot) has existed for far longer, at least into the Cretaceous. Previous islands now beneath the sea (guyots) form the Emperor Seamount Chain.[36]
All of the native Drosophila and Scaptomyza species in Hawaii have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiationwas spurred by a lack of competitionand a wide variety of niches. Although it would be possible for a single gravidfemale to colonise an island, it is more likely to have been a group from the same species.[37][38][39][40]
There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations.[41][42]
Although much is known about karyotypes at the descriptive level, and it is clear that changes in karyotype organization has had effects on the evolutionary course of many species, it is quite unclear what the general significance might be.
Cytogeneticsemploys several techniques to visualize different aspects of chromosomes:[44]
In the "classic" (depicted) karyotype, a dye, often Giemsa(G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphategroups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern.
Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximalto distalon the chromosome arms. For example, Cri du chatsyndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2).[45]
Spectral karyotyping is a molecular cytogenetictechnique used to simultaneously visualize all the pairs of chromosomesin an organism in different colors. Fluorescently-labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Because there are a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometerattached to a fluorescence microscope. Image processing software then assigns a pseudo colorto each spectrally different combination, allowing the visualization of the individually colored chromosomes.[46]
This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.
Digital Karyotyping is a technique used to quantify the DNA copy number on a genomic scale. Short sequences of DNA from specific loci all over the genome are isolated and enumerated. [47]
Chromosomal abnormalities that lead to disease in humans include:
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Some disorders arise from loss of just a piece of one chromosome, including
Chromosomal abnormalities can also occur in cancerouscells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome, a translocation mutation commonly associated with chronic myelogenous leukemiaand less often with acute lymphoblastic leukemia