Trisomy
Encyclopedia
A trisomy is a type of polysomy
in which there are three copies, instead of the normal two, of a particular chromosome
. A trisomy is a type of aneuploidy
(an abnormal number of chromosomes).
creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.
If the chromosome pairs fail to separate properly during cell division the egg or sperm may have a second copy of one of the chromosomes. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.
Trisomies are sometimes characterised as "Autosomal trisomies" (trisomies of the non-sex chromosomes) and "Sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy. Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome
, is called trisomy 21.
, but often result in miscarriage. For example, 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 types of autosomal
trisomy that survive to birth in humans are:
Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome
. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.
Trisomy of sex chromosomes can also occur:
Polysomy
Polysomy is a condition in which an organism has at least one more chromosome than normal, i.e. the number of a particular chromosome is not diploid - there may be three or more copies of the chromosome rather than the expected two copies...
in which there are three copies, instead of the normal two, of a particular chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
. A trisomy is a type of aneuploidy
Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
(an abnormal number of chromosomes).
Description and causes
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosisMeiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.
If the chromosome pairs fail to separate properly during cell division the egg or sperm may have a second copy of one of the chromosomes. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.
Terminology
"Full trisomy" means that an entire extra chromosome has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome. Randall Ausin Manges & Zachary Walker FosterTrisomies are sometimes characterised as "Autosomal trisomies" (trisomies of the non-sex chromosomes) and "Sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy. Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
, is called trisomy 21.
Human trisomy
Trisomies can occur with any chromosomeChromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
, but often result in miscarriage. For example, Trisomy 16
Trisomy 16
Trisomy 16 is a chromosomal abnormality in which there are three copies of chromosome 16 rather than two. It is the most common chromosomal cause of miscarriage during the first trimester of pregnancy....
is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage
Miscarriage
Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...
in the first trimester.
The most common types of autosomal
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
trisomy that survive to birth in humans are:
- Trisomy 21 (Down syndromeDown syndromeDown syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
) - Trisomy 18 (Edwards syndromeEdwards syndromeTrisomy 18 is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. It is named after John H. Edwards, who first described the syndrome in 1960...
) - Trisomy 13 (Patau syndromePatau syndromePatau syndrome, also known as trisomy 13 and trisomy D, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a nondisjunction of chromosomes during meiosis. Some are caused by Robertsonian translocations...
) - Trisomy 9Trisomy 9Trisomy 9 is a chromosomal disorder caused by having three copies of chromosome number 9. It can appear with or without mosaicism.-Characteristics:...
- Trisomy 8 (Warkany syndrome 2)
- Trisomy 22Trisomy 22Trisomy 22 is a chromosomal disorder in which there are three copies of chromosome 22 rather than two. It is a frequent cause of spontaneous abortion during the first trimester of pregnancy. Progression to the second trimester and livebirth are rare.This disorder is found in individuals with an...
Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome
Patau syndrome
Patau syndrome, also known as trisomy 13 and trisomy D, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a nondisjunction of chromosomes during meiosis. Some are caused by Robertsonian translocations...
. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.
Trisomy of sex chromosomes can also occur:
- XXX (Triple X syndromeTriple X syndromeTriple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The condition always produces females, with an XX pair of chromosomes, as well as an additional chromosome, resulting in the formation of XXX. A mosaic form...
)
See also
- Chromosome abnormalitiesChromosome abnormalitiesA chromosome anomaly, abnormality or aberration reflects an atypical number of chromosomes or a structural abnormality in one or more chromosomes. A Karyotype refers to a full set of chromosomes from an individual which can be compared to a "normal" Karyotype for the species via genetic testing. A...
- AneuploidyAneuploidyAneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
- KaryotypeKaryotypeA karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
- Sexual reproductionSexual reproductionSexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
- MonosomyMonosomyMonosomy is a form of aneuploidy with the presence of only one chromosome from a pair. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies.-Human monosomy:...