Karyotyping: Current perspectives in diagnosis of chromosomal disorders
Suresh Kandagal Veerabhadrappa1, Pramod Redder Chandrappa2, Seema Yadav Roodmal3, Sharan J Shetty4, Gunjiganur S Madhu Shankari2, Kumbar P Mohan Kumar2
1 Faculty of Dentistry, SEGi University, Petaling Jaya, Selangor, Malaysia
2 Department of Oral Pathology and Microbiology, College of Dental Sciences, Davangere, India
3 Department of Oral Pathology and Microbiology, AJ Institute of Dental Sciences, Mangalore, Karnataka, India
4 Department of Periodontology, Mahatma Gandhi Mission's Dental College and Hospital, Navi Mumbai, Maharashtra, India
|Date of Web Publication||9-May-2016|
Suresh Kandagal Veerabhadrappa
Faculty of Dentistry, SEGi University, No. 9, Jalan Teknologi, Taman Sains, Petaling Jaya, Kota Damansara, Selangor - 47810
Source of Support: None, Conflict of Interest: None
A majority of the genetic abnormalities are directly related to the chromosomal aberrations. Cytogenetics is the diagnostic study of the structure and properties of chromosomes and cell division, which employs various methods, one of them being "karyotyping." It refers to a procedure of photographic representation of a stained preparation in which the chromosomes are arranged in a standard manner. The development of newer techniques such as "karyotyping" has made it possible to visualize undetected chromosomal anomalies such as small portions of chromosomes and translocations of tiny parts of chromosomes to one another. Because such procedures also enabled each pair of chromosomes to be distinguished individually, it has helped to further our understanding of the chromosomal basis of certain important genetic disorders. This review article highlights the importance of "karyotyping" and its importance in the diagnosis of chromosomal disorders.
Keywords: Chromosomes, cytogenetics, genetic, translocations, karyotyping
|How to cite this article:|
Veerabhadrappa SK, Chandrappa PR, Roodmal SY, Shetty SJ, Madhu Shankari GS, Mohan Kumar KP. Karyotyping: Current perspectives in diagnosis of chromosomal disorders. Sifa Med J 2016;3:35-40
|How to cite this URL:|
Veerabhadrappa SK, Chandrappa PR, Roodmal SY, Shetty SJ, Madhu Shankari GS, Mohan Kumar KP. Karyotyping: Current perspectives in diagnosis of chromosomal disorders. Sifa Med J [serial online] 2016 [cited 2018 Mar 13];3:35-40. Available from: http://www.imjsu.org/text.asp?2016/3/2/35/182000
Living beings include variegate species that are distinct from one another. In the same species, every member has his/her own individuality, all owing to the genetic constitution of an organism - the chromosomes, the genes, and the DNA.  The blueprint for the formation and maintenance of an organism is provided by the DNA, which is packaged into chromosomes. Chromosomes are the factors, which distinguish one species from one another and which enable transmission of genetic information from one generation to the next. Chromosomes are the vehicles, which facilitate the reproduction and maintenance of a species. ,
Since a number of genetic abnormalities can be directly related to the chromosomal pattern, the characterization of chromosomes is of considerable diagnostic importance. This can be done by "cytogenetics." It is a photographic representation of a stained preparation in which the chromosomes are arranged in a standard manner and "karyotype" refers to the constitution of chromosomes of an individual.  In 1956, Tijo and Levan, Ford and Hamerton found that the normal human somatic cell contains only 46 chromosomes, and that maleness is determined by the presence of a "Y" chromosome, regardless of the number of "X" chromosomes in each cell.  The methods they used, with certain modifications, are now being used in "cytogenetic" laboratories to analyze the "karyotypes." In 1960, fibroblasts were used first for "karyotyping." 
|Procedure for Karyotyping|
I. Chromosome preparation: Source of chromosomes - any tissue with nucleated cells undergoing division can be used for chromosomal study:
- Peripheral venous blood - most commonly, the lymphocytes.
- Skin (fibroblasts), bone marrow.
- For fetal chromosome patterns - amniotic fluid cells, chorionic villi.
- 5-10 mL of heparinized venous blood is the most commonly used source. The heparin prevents coagulation, which would interfere with the later separation of lymphocytes. ,,
Culture: The blood cells are grown in a suitable culture medium containing phytohemagglutinin which, acts as a mitogen, stimulating the T-lymphocytes to divide and agglutinate the red blood cells (RBCs). The commonly used medium has 5 mL culture medium, 1 mL fetal bovine serum, and 0.2 mL phytohemagglutinin. Cultures are incubated at 37°C for 48-72 h.
Arrest of division: Mitosis is then interrupted at metaphase with spindle inhibitors such as colchicine (0.01%). Chromosome number, size, and shape at metaphase are species-specific - in nondividing cells, the chromosomes are not visible even with the aid of histologic stains for DNA or electron microscopy. During mitosis and meiosis, the chromosomes condense and become visible in the light microscope. Therefore, almost all cytogenetic work is done at metaphase. , The culture is incubated for 45 min. The contents of the vial transferred to a tube and centrifuged at 800 rpm for 5 min. Supernatant is discarded and mixed thoroughly.
Suspension in hypotonic solution: Prewarmed hypotonic saline is added to culture. This causes the RBCs to lyse. The osmotic swelling of the lymphocytes results in spreading of the chromosomes. It is incubated at 37°C for 5 min, centrifuged at 800 rpm for 5 min, and the supernatant removed.
Fixation: A freshly prepared fixative (3 parts of methanol and 1 part of glacial acetic acid) is added. Two changes of fixatives are given at intervals of 45 min.
Slide preparation: The cells resuspended in fresh fixative and slide are prepared by gently placing a drop of cell suspension on previously cooled cleaned slide and dried followed by staining. ,
II. Chromosome staining-banding techniques:
Numerous methods are available for identifying chromosomes and preparing karyotypes for diagnosis purposes. Banding patterns became the barcodes with which "cytogeneticists" can easily identify chromosomes, detect subtle deletions, inversions, insertions, translocations, fragile sites and other more complex rearrangements, and refine breakpoints. The ability to analyze chromosomes is dependent on the length of the chromosomes and how well they are fixed, spread, and stained. When a large number of cells has to be examined for clinical purposes, automatic scanning light microscopy with computer control and analysis can greatly facilitate the identification of chromosomal abnormalities.
A. General techniques: Some human chromosomes may be distinguished on morphological grounds alone, for example, the length of arms and position of primary and secondary constrictions. Autoradiography can also be used, especially for "S" phase identification and identifying chromosomes 4, 5, 13, 14, 15, 17, and 18.
Procedure: Suitable tissue preparations with a nuclear emulsion are done in a dark room, after which they are stored in the dark for several weeks and then are photographically developed and fixed. Discrete silver grains can then be seen over the sites that emit radiation; their position indicates sites of incorporation of the radioisotope. Such a preparation is termed as autoradiograph or a radioautograph. ,
Aceto-orcein method: This was the original staining technique, which permits the study of chromosome morphology. It is prepared by adding 1-2 g orecin to 45 mL hot acetic acid.
Technique: Add a few drops of stain to the prepared slide, lower coverslip, and apply gentle firm pressure with filter paper or glass rod. Remove excess stain by applying filter paper to the edge of coverslip. Chromosomes stain deep purple. This method is indelible and does not permit destaining and use of subsequent staining methods for banding. It is replaced by the banding techniques. ,
B. Banding techniques: First-banding technique was introduced by Caspersson (1969); it includes G (Giemsa) banding, Q (Quinacrine) banding, R (Reverse) banding, C (centromeric heterochromatin) banding, T (Telomeric) banding, and nucleolar organizing regions (NORs), high resolution (fine) banding. ,,
1) G (GIEMSA) BANDING: It is the most common method, which produces permanent slides that can be studied under a standard light microscope. It produces the same banding pattern as quinacrine with even greater resolution and does not necessitate the use of fluorescence microscopy. It can be used to pair and identify each of the human chromosomes accurately. ,
Prior to staining, the fixed chromosomes are treated with agents capable of denaturing chromosomal proteins such as proteolytic enzymes (trypsin, most commonly), salts, heat, detergents and urea. G-banding is most consistently produced by pretreatment of chromosomes with trypsin before staining with Giemsa. ,
Laboratory procedure: Slides for G-banding should be 1 day at room temperature or overnight at 50-60°C for optimal results.
- Incubate the slide for 20-40 s in 0.025% solution of trypsin in distilled water.
- Rinse thoroughly with phosphate-buffered saline (PBS) or distilled water.
- Stain in 4% buffered Giemsa solution for 5-10 min.
- Rinse slides in distilled water and air-dry.
- When the chromosomes are stained with Giemsa, a DNA-binding dye, after such treatment, G-bands can be seen with a light microscope.
- The G-bands constitute 300-400 alternate dark and light bands, which are characteristic for each chromosome and reflect differential chromosomal condensation. ,,,
Digital photograph of the entire metaphase spread is taken and both homologs of each chromosome pair are placed side by side in the numerical order for careful band-by-band analysis, which permits identification of relatively subtle changes in banding patterns caused by structural chromosome abnormalities [Table 1].
R-banding: A pattern that is opposite of G- or Q-banding can be produced by various means and is referred to as reverse (R-) banding. Fluorescent R-banding patterns are produced by dyes with GC base-pair affinity such as chromomycin A3, olivomycin, and mithramycin. It is produced by subjecting slides to high temperatures followed by staining with Giemsa or acridine orange. R-bands have the advantage of staining the gene-rich chromatin, thus enhancing the ability to visualize small structural rearrangements in the parts of the genome that are most likely to result in phenotypic abnormalities.
C-Banding: C-bands localize the heterochromatic regions of chromosomes. Pardue and Gall (1970) first reported C-bands when they discovered that the centromeric region of mouse chromosomes is rich in repetitive DNA sequences and stains dark with Giemsa. C-banding is also useful to show chromosomes with multiple centromeres, to study the origin of diploid molar pregnancies and true hermaphroditism, and to distinguish between donor and recipient cells in bone marrow transplantation. 
T-banding: This method involves staining the telomeric (end) regions of the chromosomes. Slides are treated with phosphate buffer or Earle's balanced salt solution and then stained using mixed Giemsa solution to produce the t-bands.
CT-banding: In this method, slides are treated with barium hydroxide to stain both the centromeric heterochromatin as well as the telomere of chromosomes to produce the CT-bands.
Nucleolar organizing region (NOR) banding: This technique stains NOR located in the satellite stalks of acrocentric chromosomes and house genes for ribosomal RNA. NOR-bands represent structural nonhistone proteins that are specifically linked to NOR and bind to ammoniacal silver. It is useful in clinical practice to study certain chromosome polymorphisms such as double satellites and also helpful to identify satellite stalks that are occasionally seen on nonacrocentric chromosomes.
|High-Resolution Banding: High|
Resolution cytogenetics provides precision in the delineation of chromosomal breakpoints.
and assignment of gene loci, greater than with earlier banding techniques. This is achieved by synchronizing the lymphocyte cultures and obtaining more number of cells in prometaphase or even prophase (increasing resolution from 500 to over 1,000 bands in a haploid genome).
Fluorescent in situ hybridization (FISH): This technique allows the visualization of chromosomal location and nuclear location of specific DNA sequences and permits the detection of specific nucleic acid sequences in morphologically preserved chromosomes. It can be performed on either metaphase or interphase cells and involves denaturing genomic DNA by using heat and formamide. ,,
After banding/staining, the chromosome component of single cells is selected on the basis of representative morphology and chromosome number and photographed under a light microscope. Photographic representation of each individual chromosome is then cut out from such a photomicrograph and arranged so as to construct a "karyotype." , Analysis is done either by looking down the microscope under oil immersion or on a photograph and involves the following steps.
- The number of chromosomes is determined: Usually the total chromosome count is determined in 15-20 cells but if mosaicism is suspected, then 30 or more cell counts are undertaken.
- Chromosomes are assembled as homologous pairs: The homologous chromosomes pairs are arranged in a decreasing order of size to construct a "karyotype."
- Detailed analysis of the banding pattern of individual chromosomes: This is performed on both members of each pair of homologs in approximately three-five metaphase spreads. ,
The banding pattern of each chromosome is specific and shown in the form of a stylized ideal karyotype known as an idiogram.
- If the total count is 46, Group G is identified to diagnose the sex of the individual.
- If the count is more or less, the precise numerical abnormality is identified. Then, any structural abnormality present is identified.
- The chromosomes are usually identified in groups starting with Group G, and then Group D, Group F, Group E, Group A, Group B, and finally Group C.
- Identification of individual chromosomes is possible on the basis of total length of the chromosome, arm ratio, position of secondary constrictions and nucleolar organizers (satellites), subdivision of chromosome into euchromatic and heterochromatic regions, and characteristic banding patterns. ,
The "karyotypes" of different groups and species are compared, and similarities in karyotypes are presumed to represent evolutionary relationships and suggest primitive or advanced feature of an organism. ,,
|Normal Human Karyotype|
A human somatic cell contains 46 chromosomes, made up of 22 pairs of autosomes and a single pair of sex chromosomes, XX in the female, XY in the male [Figure 1].
|Classification of Chromosomes for 'Karyotyping'|
Chromosomes are arranged into seven groups based on size and centromere location. The centromeres can be found in the middle of the chromosome (median), near one end (acrocentric), or in between these first two (submedian) [Figure 2].
Group A: Chromosomes 1-3 are the largest with median centromere.
Group B: Chromosomes 4-5 are large with submedian centromere.
Group C: Chromosomes 6-12 are medium-sized with submedian centromere.
Group D: Chromosomes 13-15 are medium-sized with acrocentric centromere.
Group E: Chromosomes 16-18 are short with median or submedian centromere.
Group F: Chromosomes 19-20 are short with median centromere.
Group G: Chromosomes 21-22 are very short with acrocentric centromere.
Chromosome X is similar to group C.
Chromosome Y is similar to group G
Applications of "karyotyping" includes: ,
- Cytogenetic aspect of neonatally discovered congenital abnormalities and prenatal diagnosis.
- Mental retardation and neuropsychiatric disorders.
- Reproductive failure, endocrinology, gynecology, urology.
- Malignancy: Hematology and oncology.
- Mutagenesis: Environmental medicine and industrial medicine. ,
- "Karyotyping" in chromosomal abnormalities: Approximately 60% of all spontaneous abortions and 1 out of 200 newborns have some form of chromosomal abnormality, which can be detected by "karyotyping" pre/postnatally.
- Structural Abnormalities: Structural chromosomal rearrangements result from chromosome breakage, with/without subsequent reunion in a different configuration. They can be balanced generally harmless with few effects and unbalanced serious clinical effects. Commonly identified structural abnormalities include translocation, deletion, inversion, ring chromosomes, isochromosomes, mosaicism, and chimerism [Table 2].
Syndromes associated with chromosome abnormalities [Table 3].
The development of newer techniques such as "karyotyping" has made it possible to visualize undetected chromosomal anomalies such as small portions of chromosomes and translocations of tiny parts of chromosomes to one another. Because such procedures also enabled each pair of chromosomes to be distinguished individually, it has helped to further our understanding of chromosomal basis of certain important genetic disorders.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]
"Cytotype" redirects here. For other uses, see Cytotype (disambiguation).
A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryoticcell. The term is also used for the complete set of chromosomes in a species or in an individual organism 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. The preparation and study of karyotypes is part of cytogenetics.
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).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) 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) 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 (1922). The subsequent history of the concept can be followed in the works of C. D. Darlington and Michael JD White.
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 XX/XOsex determination mechanism.Painter in 1922 was not certain whether the diploid of humans was 46 or 48, at first favoring 46, but revised his opinion from 46 to 48, and he correctly insisted on humans having an XX/XY system. Considering the techniques of the time, these results were remarkable.
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 was responsible for finding the approach:
- Using cells in tissue culture
- Pretreating cells in a 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.
The work took place in 1955, and was published in 1956. The karyotype of humans includes only 46 chromosomes. Rather interestingly, 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.
Observations on karyotypes
The study of karyotypes is made possible by staining. Usually, a suitable dye, such as Giemsa, is applied after 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. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn fetus can be determined by observation of interphase cells (see amniotic centesis and Barr body).
Six different characteristics of karyotypes are usually observed and compared:
- 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. Satellites are small bodies attached to a chromosome by a thin thread.
- Differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin. Heterochromatin is packed tighter. Heterochromatin consists mainly of genetically inactive and repetitive DNA sequences as well as containing a larger amount of Adenine-Thymine pairs. Euchromatin is usually under active transcription and stains much lighter as it has less affinity for the giemsa stain. Euchromatin regions contain larger amounts of Guanine-Cytosine pairs. The staining technique using giemsa staining is called G banding and therefore produces the typical "G-Bands".
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 soma (between gametes and the rest of the body),
- between members of a population (chromosome polymorphism),
- in geographic specialization, and
- in mosaics or otherwise abnormal individuals.
The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes (allosomes). Normal karyotypes for females contain two X chromosomes and are denoted 46,XX; males have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Diversity and evolution of karyotypes
Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable. There is variation between species in chromosome number, and in detailed organization, despite their construction from the same macromolecules. This variation provides the basis for a range of studies in evolutionary cytology.
In some cases there is even significant variation within species. In a review, Godfrey and Masters 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.
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.
We have a very poor understanding of the causes of karyotype evolution, despite many careful investigations ... the general significance of karyotype evolution is obscure.
— Maynard Smith
Changes during development
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.
- Chromosome elimination. In some species, as in many sciarid flies, entire chromosomes are eliminated during development.
- Chromatin diminution (founding father: Theodor Boveri). In this process, found in some copepods and roundworms such as Ascaris suum, portions of the chromosomes are cast away in particular cells. This process is a carefully organised genome rearrangement where new telomeres are constructed and certain heterochromatin regions are lost. In A. suum, all the somatic cell precursors undergo chromatin diminution.
- X-inactivation. The inactivation of one X chromosome takes place during the early development of mammals (see Barr body and dosage compensation). In placental mammals, the inactivation is random as between the two Xs; thus the mammalian female is a mosaic in respect of her X chromosomes. In marsupials it is always the paternal X which is inactivated. In human females some 15% of somatic cells escape inactivation, and the number of genes affected on the inactivated X chromosome varies between cells: in fibroblast cells up about 25% of genes on the Barr body escape inactivation.
Number of chromosomes in a set
A spectacular example of variability between closely related species is the muntjac, which was investigated by Kurt Benirschke and 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.
They simply could not believe what they saw ... They kept quiet for two or three years because they thought something was wrong with their tissue culture ... But when they obtained a couple more specimens they confirmed [their findings].
— Hsu p. 73-4
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 haploid n = 1; and an ant: Myrmecia pilosula. The high record would be somewhere amongst the ferns, with the adder's tongue fern Ophioglossum ahead with an average of 1262 chromosomes. Top score for animals might be the shortnose sturgeonAcipenser brevirostrum at 372 chromosomes. The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.
The fundamental number, FN, of a karyotype is the number of visible major chromosomal arms per set of chromosomes. Thus, FN ≤ 2 x 2n, the difference depending on the number of chromosomes considered single-armed (acrocentric or telocentric) present. Humans have FN = 82, due to the presence of five acrocentric chromosome pairs: 13, 14, 15, 21, and 22 (the human Y chromosome is also acrocentric). The fundamental autosomal number or autosomal fundamental number, FNa or AN, of a karyotype is the number of visible major chromosomal arms per set of autosomes (non-sex-linked chromosomes).
Ploidy is the number of complete sets of chromosomes in a cell.
- Polyploidy, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants. It has been of major significance in plant evolution according to Stebbins. The proportion of flowering plants which are polyploid was estimated by Stebbins to be 30–35%, but in grasses the average is much higher, about 70%. Polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants.
Polyploidy in animals is much less common, but it has been significant in some groups.
Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid.
- Haplo-diploidy, where one sex is diploid, and the other haploid. It is a common arrangement in the Hymenoptera, and in some other groups.
- Endopolyploidy occurs when in adult differentiated tissues the cells have ceased to divide by mitosis, but the nuclei contain more than the original somatic number of chromosomes. In the endocycle (endomitosis or endoreduplication) chromosomes in a 'resting' nucleus undergo reduplication, the daughter chromosomes separating from each other inside an intactnuclear membrane.
In many instances, endopolyploid nuclei contain tens of thousands of chromosomes (which cannot be exactly counted). The cells do not always contain exact multiples (powers of two), which is why the simple definition 'an increase in the number of chromosome sets caused by replication without cell division' is not quite accurate.
This process (especially studied in insects and some higher plants such as maize) may be a developmental strategy for increasing the productivity of tissues which are highly active in biosynthesis.
The phenomenon occurs sporadically throughout the eukaryote kingdom from protozoa to humans; it is diverse and complex, and serves differentiation and morphogenesis in many ways.
- See palaeopolyploidy for the investigation of ancient karyotype duplications.
Aneuploidy is the condition in which the chromosome number in the cells is not the typical number for the species. This would give rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost. Abnormalities in chromosome number usually cause a defect in development. Down syndrome and Turner syndrome are examples of this.
Aneuploidy may also occur within a group of closely related species. Classic examples in plants are the genus Crepis, where the gametic (= haploid) numbers form the series x = 3, 4, 5, 6, and 7; and Crocus, where every number from x = 3 to x = 15 is represented by at least one species. Evidence of various kinds shows that trends of evolution have gone in different directions in different groups. Closer to home, the great apes have 24x2 chromosomes whereas humans have 23x2. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.
Some species are polymorphic for different chromosome structural forms. The structural variation may be associated with different numbers of chromosomes in different individuals, which occurs in the ladybird beetle Chilocorus stigma, some mantids of the genus Ameles, the European shrew Sorex araneus. There is some evidence from the case of the molluscThais lapillus (the dog whelk) on the Brittany coast, that the two chromosome morphs are adapted to different habitats.
The detailed study of chromosome banding in insects with polytene chromosomes can reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilids by Hampton L. Carson.
In about 6,500 sq mi (17,000 km2), 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-Ar dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll, which can be dated to 30 mya. The archipelago itself (produced by the Pacific plate moving 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.
All of the native Drosophila and Scaptomyza species in Hawaiʻi have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiation was spurred by a lack of competition and a wide variety of niches. Although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species.
There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations.
Chromosomes display a banded pattern when treated with some stains. Bands are alternating light and dark stripes that appear along the lengths of chromosomes. Unique banding patterns are used to identify chromosomes and to diagnose chromosomal aberrations, including chromosome breakage, loss, duplication, translocation or inverted segments. A range of different chromosome treatments produce a range of banding patterns: G-bands, R-bands, C-bands, Q-bands, T-bands and NOR-bands.
Depiction of karyotypes
Types of banding
Cytogenetics employs several techniques to visualize different aspects of chromosomes:
- G-banding is obtained with Giemsa stain following digestion of chromosomes with trypsin. It yields a series of lightly and darkly stained bands — the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300–400 bands in a normal, human genome.
- R-banding is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).
- C-banding: Giemsa binds to constitutive heterochromatin, so it stains centromeres.The name is derived from centromeric or constitutive heterochromatin. The preparations undergo alkaline denaturation prior to staining leading to an almost complete depurination of the DNA. After washing the probe the remaining DNA is renatured again and stained with Giemsa solution consisting of methylene azure, methylene violet, methylene blue, and eosin. Heterochromatin binds a lot of the dye, while the rest of the chromosomes absorb only little of it. The C-bonding proved to be especially well-suited for the characterization of plant chromosomes.
- Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding.They can be recognized by a yellow fluorescence of differing intensity. Most part of the stained DNA is heterochromatin. Quinacrin (atebrin) binds both regions rich in AT and in GC, but only the AT-quinacrin-complex fluoresces. Since regions rich in AT are more common in heterochromatin than in euchromatin, these regions are labelled preferentially. The different intensities of the single bands mirror the different contents of AT. Other fluorochromes like DAPI or Hoechst 33258 lead also to characteristic, reproducible patterns. Each of them produces its specific pattern. In other words: the properties of the bonds and the specificity of the fluorochromes are not exclusively based on their affinity to regions rich in AT. Rather, the distribution of AT and the association of AT with other molecules like histones, for example, influences the binding properties of the fluorochromes.
- T-banding: visualize telomeres.
- Silver staining: Silver nitrate stains the nucleolar organization region-associated protein. This yields a dark region where the silver is deposited, denoting the activity of rRNA genes within the NOR.
Classic karyotype cytogenetics
In the "classic" (depicted) karyotype, a dye, often Giemsa(G-banding), less frequently mepacrine (quinacrine), is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups 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 proximal to distal on the chromosome arms. For example, Cri du chat syndrome 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 p15.2 (the locus on the chromosome), which is written as 46,XX,del(5)(p15.2).
Multicolor FISH (mFISH) and spectral karyotype (SKY technique)
Multicolor FISH and the older spectral karyotyping are molecular cytogenetic techniques used to simultaneously visualize all the pairs of chromosomes in 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. Fluorophore combinations are captured and analyzed by a fluorescence microscope using up to 7 narrow-banded fluorescence filters or, in the case of spectral karyotyping, by using an interferometer attached to a fluorescence microscope. In the case of an mFISH image, every combination of fluorochromes from the resulting original images is replaced by a pseudo color in a dedicated image analysis software. Thus, chromosomes or chromosome sections can be visualized and identified, allowing for the analysis of chromosomal rearrangements. In the case of spectral karyotyping, image processing software assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.
Multicolor FISH 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. This method is also known as virtual karyotyping.
Main article: Chromosome abnormalities
Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in derivative chromosome, translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells.
Chromosomal abnormalities that lead to disease in humans include
- Turner syndrome results from a single X chromosome (45,X or 45,X0).
- Klinefelter syndrome, the most common male chromosomal disease, otherwise known as 47,XXY, is caused by an extra X chromosome.
- Edwards syndrome is caused by trisomy (three copies) of chromosome 18.
- Down syndrome, a common chromosomal disease, is caused by trisomy of chromosome 21.
- Patau syndrome is caused by trisomy of chromosome 13.
- Trisomy 9, believed to be the 4th most common trisomy, has many long lived affected individuals but only in a form other than a full trisomy, such as trisomy 9p syndrome or mosaic trisomy 9. They often function quite well, but tend to have trouble with speech.
- Also documented are trisomy 8 and trisomy 16, although they generally do not survive to birth.
Some disorders arise from loss of just a piece of one chromosome, including
- Cri du chat (cry of the cat), from a truncated short arm on chromosome 5. The name comes from the babies' distinctive cry, caused by abnormal formation of the larynx.
- 1p36 Deletion syndrome, from the loss of part of the short arm of chromosome 1.
- Angelman syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the maternal genes, example of imprinting disorder.
- Prader-Willi syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the paternal genes, example of imprinting disorder.
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome, a translocation mutation commonly associated with chronic myelogenous leukemia and less often with acute lymphoblastic leukemia.
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Fusion of ancestral chromosomes left distinctive remnants of telomeres, and a vestigial centromere