Chromosome

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Diagram of a replicated and condensed metaphase eukaryotic chromosome. (1) Chromatid - one of the two identical parts of the chromosome after S phase. (2) Centromere - the point where the two chromatids touch. (3) Short arm (p). (4) Long arm (q). Chromosome.svg
Diagram of a replicated and condensed metaphase eukaryotic chromosome. (1) Chromatid  – one of the two identical parts of the chromosome after S phase. (2) Centromere  – the point where the two chromatids touch. (3) Short arm (p). (4) Long arm (q).

A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity. [1] [2] These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation. [3]

Contents

Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form). [4] Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study. [5] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation. [6]

Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.

Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

Etymology

The word chromosome ( /ˈkrməˌsm,-ˌzm/ [7] [8] ) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes. [9] The term was coined by the German anatomist Heinrich Wilhelm Waldeyer, [10] referring to the term chromatin, which was introduced by Walther Flemming, the discoverer of cell division.

Some of the early karyological terms have become outdated. [11] [12] For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state. [13]

History of discovery

Walter sutton.jpg
Theodor Boveri.jpg
Walter Sutton (left) and Theodor Boveri (right) independently developed the chromosome theory of inheritance in 1902.

The German scientists Schleiden, [5] Virchow and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. [14]

In a series of experiments beginning in the mid-1880s, Theodor Boveri gave definitive contributions to elucidating that chromosomes are the vectors of heredity, with two notions that became known as ‘chromosome continuity’ and ‘chromosome individuality’. [15]

Wilhelm Roux suggested that each chromosome carries a different genetic configuration, and Boveri was able to test and confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson, Stevens, and Painter actually worked with him). [16]

In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory (the names are sometimes reversed). [17] Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Morgan, all of a rather dogmatic turn of mind. Eventually, complete proof came from chromosome maps in Morgan's own lab. [18]

The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted 24 pairs, which would mean 48 chromosomes. His error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio. [19]

Prokaryotes

The prokaryotes  – bacteria and archaea  – typically have a single circular chromosome, but many variations exist. [20] The chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola [21] and Candidatus Tremblaya princeps , [22] to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum . [23] Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi , the cause of Lyme disease, containing a single linear chromosome. [24]

Structure in sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins. [25] The genes in prokaryotes are often organized in operons, and do not usually contain introns, unlike eukaryotes.

DNA packaging

Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid. [26] [27] The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. This structure is, however, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome. [28] In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes. [29] [30]

Certain bacteria also contain plasmids or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. [5] In prokaryotes (see nucleoids) and viruses, [31] the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like proteins.

Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and replication.

Eukaryotes

Organization of DNA in a eukaryotic cell Eukaryote DNA-en.svg
Organization of DNA in a eukaryotic cell

Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called chromatin. Chromatin contains the vast majority of the DNA of an organism, but a small amount inherited maternally, can be found in the mitochondria. It is present in most cells, with a few exceptions, for example, red blood cells.

Histones are responsible for the first and most basic unit of chromosome organization, the nucleosome.

Eukaryotes (cells with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes.

The major structures in DNA compaction: DNA, the nucleosome, the 10 nm "beads-on-a-string" fibre, the 30 nm fibre and the metaphase chromosome. Chromatin Structures.png
The major structures in DNA compaction: DNA, the nucleosome, the 10 nm "beads-on-a-string" fibre, the 30 nm fibre and the metaphase chromosome.

In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around histones (structural proteins), forming a composite material called chromatin.

Interphase chromatin

The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30 nm fibres [32] Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers. [32] Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription. [32]

Heterochromatin vs. euchromatin Heterochromatin vs. euchromatin.svg
Heterochromatin vs. euchromatin

During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished:

Metaphase chromatin and division

Human chromosomes during metaphase HumanChromosomesChromomycinA3.jpg
Human chromosomes during metaphase
Stages of early mitosis in a vertebrate cell with micrographs of chromatids Stages of early mitosis in a vertebrate cell with micrographs of chromatids.svg
Stages of early mitosis in a vertebrate cell with micrographs of chromatids

In the early stages of mitosis or meiosis (cell division), the chromatin double helix become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. The loops of 30-nm chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about 10,000 fold. [32]

The chromosome scaffold, which is made of proteins such as condensin, TOP2A and KIF4, [33] plays an important role in holding the chromatin into compact chromosomes. Loops of 30 nm structure further condense with scaffold into higher order structures. [34]

This highly compact form makes the individual chromosomes visible, and they form the classic four-arm structure, a pair of sister chromatids attached to each other at the centromere. The shorter arms are called p arms (from the French petit, small) and the longer arms are called q arms (q follows p in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for queue meaning tail in French [35] ). This is the only natural context in which individual chromosomes are visible with an optical microscope.

Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops. [36]

During mitosis, microtubules grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called kinetochores, one of which is present on each sister chromatid. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.

Human chromosomes

Chromosomes in humans can be divided into two types: autosomes (body chromosome(s)) and allosome (sex chromosome(s)). Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database. [37] Number of genes is an estimate, as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.

Chromosome Genes [38] Total base pairs  % of basesSequenced base pairs [39] % sequenced base pairs
1 2000247,199,7198.0224,999,71991.02%
2 1300242,751,1497.9237,712,64997.92%
3 1000199,446,8276.5194,704,82797.62%
4 1000191,263,0636.2187,297,06397.93%
5 900180,837,8665.9177,702,76698.27%
6 1000170,896,9935.5167,273,99397.88%
7 900158,821,4245.2154,952,42497.56%
8 700146,274,8264.7142,612,82697.50%
9 800140,442,2984.6120,312,29885.67%
10 700135,374,7374.4131,624,73797.23%
11 1300134,452,3844.4131,130,85397.53%
12 1100132,289,5344.3130,303,53498.50%
13 300114,127,9803.795,559,98083.73%
14 800106,360,5853.588,290,58583.01%
15 600100,338,9153.381,341,91581.07%
16 80088,822,2542.978,884,75488.81%
17 120078,654,7422.677,800,22098.91%
18 20076,117,1532.574,656,15598.08%
19 150063,806,6512.155,785,65187.43%
20 50062,435,9652.059,505,25495.31%
21 20046,944,3231.534,171,99872.79%
22 50049,528,9531.634,893,95370.45%
X (sex chromosome) 800154,913,7545.0151,058,75497.51%
Y (sex chromosome) 200 [40] 57,741,6521.925,121,65243.51%
Total21,0003,079,843,747100.02,857,698,56092.79%

Number in various organisms

In eukaryotes

The number of chromosomes in eukaryotes is highly variable (see table). In fact, chromosomes can fuse or break and thus evolve into novel karyotypes. Chromosomes can also be fused artificially. For example, the 16 chromosomes of yeast have been fused into one giant chromosome and the cells were still viable with only somewhat reduced growth rates. [41]

The tables below give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, most eukaryotes are diploid, like humans who have 22 different types of autosomes, each present as two homologous pairs, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosome types, such as bread wheat, which is hexaploid and has six copies of seven different chromosome types – 42 chromosomes in total.

Chromosome numbers in some plants
Plant Species#
Arabidopsis thaliana (diploid) [42] 10
Rye (diploid) [43] 14
Einkorn wheat (diploid) [44] 14
Maize (diploid or palaeotetraploid) [45] 20
Durum wheat (tetraploid) [44] 28
Bread wheat (hexaploid) [44] 42
Cultivated tobacco (tetraploid) [46] 48
Adder's tongue fern (polyploid) [47] approx. 1,200
Chromosome numbers (2n) in some animals
Species#
Indian muntjac 7
Common fruit fly 8
Pill millipede (Arthrosphaera fumosa) [48] 30
Earthworm (Octodrilus complanatus) [49] 36
Tibetan fox 36
Domestic cat [50] 38
Domestic pig 38
Laboratory mouse [51] [52] 40
Laboratory rat [52] 42
Rabbit (Oryctolagus cuniculus) [53] 44
Syrian hamster [51] 44
Guppy (poecilia reticulata) [54] 46
Human [55] 46
Hares [56] [57] 48
Gorillas, chimpanzees [55] 48
Domestic sheep 54
Garden snail [58] 54
Silkworm [59] 56
Elephant [60] 56
Cow 60
Donkey 62
Guinea pig [61] 64
Horse 64
Dog [62] 78
Hedgehog 90
Goldfish [63] 100–104
Kingfisher [64] 132
Chromosome numbers in other organisms
SpeciesLarge
Chromosomes
Intermediate
Chromosomes
Microchromosomes
Trypanosoma brucei 116≈100
Domestic pigeon
(Columba livia domestica) [65]
1859–63
Chicken [66] 82 sex chromosomes60

Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell.

The 23 human chromosome territories during prometaphase in fibroblast cells PLoSBiol3.5.Fig1bNucleus46Chromosomes.jpg
The 23 human chromosome territories during prometaphase in fibroblast cells

Asexually reproducing species have one set of chromosomes that are the same in all body cells. However, asexual species can be either haploid or diploid.

Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father. Gametes, reproductive cells, are haploid [n]: They have one set of chromosomes. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.

Some animal and plant species are polyploid [Xn]: They have more than two sets of homologous chromosomes. Plants important in agriculture such as tobacco or wheat are often polyploid, compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more-common pasta and bread wheat types are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat. [67]

In prokaryotes

Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. [68] For example, Buchnera , a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell. [69] However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present. [70] Plasmids and plasmid-like small chromosomes are, as in eukaryotes, highly variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid – fast division causes high copy number.

Karyotype

Karyogram of a human male NHGRI human male karyotype.png
Karyogram of a human male

In general, the karyotype is the characteristic chromosome complement of a eukaryote species. [71] The preparation and study of karyotypes is part of cytogenetics.

Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization. In some cases, there is significant variation within species. Often there is:

1. variation between the two sexes
2. variation between the germ-line and soma (between gametes and the rest of the body)
3. variation between members of a population, due to balanced genetic polymorphism
4. geographical variation between races
5. mosaics or otherwise abnormal individuals.

Also, variation in karyotype may occur during development from the fertilized egg.

The technique of determining the karyotype is usually called karyotyping. Cells can be locked part-way through division (in metaphase) in vitro (in a reaction vial) with colchicine. These cells are then stained, photographed, and arranged into a karyogram, with the set of chromosomes arranged, autosomes in order of length, and sex chromosomes (here X/Y) at the end.

Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males.

History and analysis techniques

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/XO sex determination mechanism. [72] Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46. [73] He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system. [74]

New techniques were needed to definitively solve the problem:

  1. Using cells in culture
  2. Arresting mitosis in metaphase by a solution of colchicine
  3. Pretreating cells in a hypotonic solution 0.075 M KCl, which swells them and spreads the chromosomes
  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.

It took until 1954 before the human diploid number was confirmed as 46. [75] [76] Considering the techniques of Winiwarter and Painter, their results were quite remarkable. [77] Chimpanzees, the closest living relatives to modern humans, have 48 chromosomes as do the other great apes: in humans two chromosomes fused to form chromosome 2.

Aberrations

In Down syndrome, there are three copies of chromosome 21. Chromosome 21.png
In Down syndrome, there are three copies of chromosome 21.

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans, such as Down syndrome, although most aberrations have little to no effect. Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of bearing a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, called aneuploidy, may be lethal or may give rise to genetic disorders. [78] Genetic counseling is offered for families that may carry a chromosome rearrangement.

The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Human examples include:

Sperm aneuploidy

Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa. [82] In particular, risk of aneuploidy is increased by tobacco smoking, [83] [84] and occupational exposure to benzene, [85] insecticides, [86] [87] and perfluorinated compounds. [88] Increased aneuploidy is often associated with increased DNA damage in spermatozoa.

See also

Notes and references

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Related Research Articles

Autosome Any chromosome other than a sex chromosome

An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures. The DNA in autosomes is collectively known as atDNA or auDNA.

Centromere Specialized DNA sequence of a chromosome that links a pair of sister chromatids

The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first thought to be genetic loci that direct the behavior of chromosomes.

Meiosis Type of cell division in sexually-reproducing organisms used to produce gametes

Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.

Ploidy Number of sets of chromosomes in a cell

Ploidy is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair, which chromosomes naturally exist as. Somatic cells, tissues, and individual organisms can be described according to the number of sets of chromosomes present : monoploid, diploid, triploid, tetraploid, pentaploid, hexaploid, heptaploid or septaploid, etc. The generic term polyploid is often used to describe cells with three or more chromosome sets.

Karyotype Photographic display of total chromosome complement in a cell

A karyotype is a preparation of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, sorted by length, centromere location and other features. and for a test that detects this complement or counts the number of chromosomes. Karyotyping is the process by which a karyotype is prepared from photographs of chromosomes, in order to determine the chromosome complement of an individual, including the number of chromosomes and any abnormalities.

Aneuploidy Presence of an abnormal number of chromosomes in a cell

Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell.

Cytogenetics

Cytogenetics is essentially a branch of genetics, but is also a part of cell biology/cytology, that is concerned with how the chromosomes relate to cell behaviour, particularly to their behaviour during mitosis and meiosis. Techniques used include karyotyping, analysis of G-banded chromosomes, other cytogenetic banding techniques, as well as molecular cytogenetics such as fluorescent in situ hybridization (FISH) and comparative genomic hybridization (CGH).

Nondisjunction Failure to separate properly during cell division

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

Chromosomal translocation

In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translocation is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes. Two detached fragments of two different chromosomes are switched. Robertsonian translocation occurs when two non-homologous chromosomes get attached, meaning that given two healthy pairs of chromosomes, one of each pair "sticks" together.

Human genetics Study of inheritance as it occurs in human beings

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genetics, a discipline of biology, is the science of heredity and variation in living organisms.

Isochromosome

An isochromosome is an unbalanced structural abnormality in which the arms of the chromosome are mirror images of each other. The chromosome consists of two copies of either the long (q) arm or the short (p) arm because isochromosome formation is equivalent to a simultaneous duplication and deletion of genetic material. Consequently, there is partial trisomy of the genes present in the isochromosome and partial monosomy of the genes in the lost arm.

Polysomy Abnormal multiples of one or more chromosomes

Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most eukaryotic species are diploid, meaning they have two sets of chromosomes, whereas prokaryotes are haploid, containing a single chromosome in each cell. Aneuploids possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A karyotype is the set of chromosomes in an organism and the suffix -somy is used to name aneuploid karyotypes. This is not to be confused with the suffix -ploidy, referring to the number of complete sets of chromosomes.

A dicentric chromosome is an abnormal chromosome with two centromeres. It is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments and the formation of dicentric fragments. The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation and paracentric inversion. Dicentric chromosomes have important roles in the mitotic stability of chromosomes and the formation of pseudodicentric chromosomes. Their existence has been linked to certain natural phenomena such as irradiation and have been documented to underlie certain clinical syndromes, notably Kabuki syndrome. The formation of dicentric chromosomes and their implications on centromere function are studied in certain clinical cytogenetics laboratories.

A chromosome abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where there is an atypical number of chromosomes, or as structural abnormalities, where one or more individual chromosomes are altered. Chromosome mutation was formerly used in a strict sense to mean a change in a chromosomal segment, involving more than one gene. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. Chromosome abnormalities may be detected or confirmed by comparing an individual's karyotype, or full set of chromosomes, to a typical karyotype for the species via genetic testing.

The following outline is provided as an overview of and topical guide to genetics:

Neocentromere

Neocentromeres are new centromeres that form at a place on the chromosome that is usually not centromeric. They typically arise due to disruption of the normal centromere. These neocentromeres should not be confused with “knobs”, which were also described as “neocentromeres” in maize in the 1950s. Unlike most normal centromeres, neocentromeres do not contain satellite sequences that are highly repetitive but instead consist of unique sequences. Despite this, most neocentromeres are still able to carry out the functions of normal centromeres in regulating chromosome segregation and inheritance. This raises many questions on what is necessary versus what is sufficient for constituting a centromere.

Tetrasomy X Chromosomal disorder with 4 X chromosomes

Tetrasomy X, also known as 48,XXXX, is a chromosomal disorder in which a female has four, rather than two, copies of the X chromosome. It is associated with intellectual disability of varying severity, characteristic "coarse" facial features, heart defects, and skeletal anomalies such as increased height, clinodactyly, and radioulnar synostosis. Tetrasomy X is a rare condition, with few medically recognized cases; it is estimated to occur in approximately 1 in 50,000 females.

Trisomy X Chromosome disorder in women

Trisomy X, also known as triple X syndrome and characterized by the karyotype 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 women but it is rarely diagnosed; fewer than 10% of those with the condition know they have it.