Aneuploidy

# Overview
Aneuploidy is a change in the number of chromosomes that can lead to a chromosomal disorder.  The most recognized forms of aneuploidy are the trisomy diseases of Down syndrome and Edwards syndrome. Trisomy may not necessarily be present in all cells in an individual.  It may be detected in just a specific tissue or within different cells in a tissue.  When the presence of chromosomal abnormalities occurs differentially within an individual, it is called chromosomal mosaicism.  In general, as we would expect, individuals who are mosaic for a chromosome change tend to have a less severe form of the syndrome present than full trisomy individuals.  Critical examples of mosaicism are found in leukemia cases, specifically; chronic lymphocytic leukemia (CLL) which is a trisomy of chromosome 12 and acute myeloid leukemia (AML) prognosis which is a trisomy of chromosome 8.  Aneuploidy is common in cancerous cells.

# Commonly observed forms

## Monosomy
Monosomy is the presence of only one chromosome from a pair in a cell's nucleus. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies.

### Human monosomy
Human genetic disorders arising from monosomy are:
- X0-Only one X chromosome instead of the usual two (XX) seen in a normal female, also known as Turner syndrome
- Cri du chat syndrome -- (French for "cry of the cat" after the distinctive noise by affected persons' malformed larynx) a partial monosomy caused by a deletion of the end of the short p (from the word petit, French for small) arm of chromosome 5
- 1p36 Deletion Syndrome -- a partial monosomy caused by a deletion at the end of the short p arm of chromosome 1

## Disomy
A disomy is the presence of a pair of chromosomes. For diploid organisms, such as humans, it is the normal condition.  For organisms that are normally triploid or above, disomy is an aneuploidy.  It can also refer to cells that are normally haploid, such as gametes.
Uniparental disomy, the disomy refers to two copies of the chromosome from one of the parents (with no contribution from the other parent).

## Trisomy
A trisomy is the presence of three, instead of the normal two, chromosomes of a particular numbered type in an organism. Thus the presence of an extra chromosome 21 is called trisomy 21.
Full trisomy of an individual occurs due to non-disjunction during meiosis I or meiosis II of gametogenesis resulting in 24 vice 23 chromosomes in a reproductive cell (sperm or egg).  Thus, after fertilization, the resulting fetus has 47 chromosomes vice the typical 46.  The most common forms of autosomal trisomy are trisomy of chromosome 21 which results in Down syndrome and trisomy of chromosome 18 which results in Edwards syndrome.  In rare cases, a fetus with trisomy of chromosome 13 can survive.  Trisomy 13 is called Patau syndrome.  Autosomal trisomy is frequently associated with severe congenital abnormalities, mental retardation and shortened life expectancy.
A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes, or if one of the chromosomes has two copies of part of its chromosome. A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.

### Human trisomy
A trisomy can occur with any chromosome.  Most trisomies, like most other abnormalities in chromosome number, result in distinctive and serious birth defects. Mostly, the causes are autosomal and sex chromosomal nondisjunction.  Most trisomies result in spontaneous abortion; the most common types that survive to birth in humans are:
Autosomal nondisjunction
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Trisomy 12 (A prognostic indicator of Chronic Lymphocytic Leukemia)
- Trisomy 9
- Trisomy 8 (Warkany syndrome 2)
Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common trisomy in viable births is Trisomy 21.
Sex-chromosomal nondisjunction
Aneuploidy of sex chromosomes can also occur.  The presence of extra X chromosome(s) causes Klinefelter syndrome in men and Triple X syndrome in women, while monosomy X (45, X) gives rise to women with Turner syndrome.
- XXX (Triple X syndrome)
- XXY (Klinefelter's syndrome)
- XYY (XYY syndrome)

## Tetrasomy and pentasomy
A tetrasomy and a pentasomy are the presence of a respectively four or five copies of a chromosome. Although very rare, reported examples of tetrasomy and pentasomy in humans include the karyotypes XXXX (XXXX syndrome), XXXY, XXYY, XYYY, XXXXX, XXXXY, XXXYY, XXYYY and XYYYY.

# Mechanisms
- Nondisjunction usually occurs as the result of a weakened mitotic checkpoint, as these checkpoints tend to arrest or delay cell division until all components of the cell are ready to enter the next phase. If a checkpoint is weakened, the cell may fail to 'notice' that a chromosome pair is not lined up on the mitotic plate, for example. In such a case, most chromosomes would separate normally (with one chromatid ending up in each cell), while others could fail to separate at all. This would generate a daughter cell lacking a copy and a daughter cell with an extra copy.
- Completely inactive mitotic checkpoints may cause non-disjunction at multiple chromosomes, possibly all. Such a scenario could result in each daughter cell possessing a disjunct set of genetic material.
- Merotelic attachment occurs when one kinetochore is attached to both mitotic spindle poles. One daughter cell would have a normal complement of chromosomes, the second would lack one. A third daughter cell may end up with the 'missing' chromosome.
- Multipolar spindle: more than two spindle poles form. Such a mitotic division would result in one daughter cell for each spindle pole; each cell may possess an unpredictable complement of chromosomes.
- Monopolar spindle: only a single spindle pole forms. This produces a single daughter cell with its copy number doubled.
- A tetraploid intermediate may be produced as the end result of the monopolar spindle mechanism. In such a case, the cell has double the copy number of a normal cell, and produces double the number of spindle poles as well. This results in four daughter cells with an unpredictable complement of chromosomes, but in the normal copy number.

# Detection
Several prenatal tests can be performed on an expectant mother to detect aneuploidy in the fetus.  Fetal cells from the amniotic fluid or chorionic villus are collected and analyzed by one of several techniques which include Fluorescence In Situ Hybridization (FISH) karyotyping,  Quantitative Polymerase Chain Reaction (PCR) of Short Tandem Repeats, Quantitative Fluorescence PCR (QF-PCR), Quantitative Real-time PCR (RT-PCR) dosage analysis, Quantitative Mass Spectrometry of Single Nucleotide Polymorphisms, and Comparative Genomic Hybridization (CGH).  A program for detecting aneuploidy by short tandem repeat fragments is SoftGenetics GeneMarker software.
Pregnant women over the age of 35 are screened for trisomy caused syndromes in the fetus because the risk of these syndromes increases as the mother ages.  During the procedure whereby cells are collected for analysis, maternal cells can be mixed in with fetal cells.  This phenomenon is called Maternal Cell Contamination (MCC).

# Sources
- This article incorporates public domain text from The U.S. National Library of Medicine.
- Sex chromosome tetrasomy and pentasomy. PMID 7567329