A trisomy is a type of polysomy in which there are three instances of a particular

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

, instead of the normal two. A trisomy is a type of

aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell (biology), cell, for example a human somatic (biology), somatic cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more plo ...

(an abnormal number of chromosomes).

Description and causes

Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one of each type of chromosome inherited from each parent. In such organisms,

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

creates gamete cells (eggs or sperm) having only one set of chromosomes. The number of chromosomes is different for different species, with humans having 46 chromosomes (23 pairs) and human gametes 23 chromosomes. If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes ( non-disjunction). If such a gamete is fertilized with a normal gamete, the resulting embryo may have a total of three copies of the chromosome.

Terminology

The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2''n''+1 if one chromosome shows trisomy, 2''n''+1+1 if two show trisomy, etc. *"Full trisomy", also called "primary trisomy", means that an entire extra chromosome has been copied. *"Partial trisomy" means that there is an extra copy of part of a chromosome. *"Secondary trisomy" - the extra chromosome has quadruplicated arms (the arms are identical; it is an "isochromosome"). *"Tertiary trisomy" - the extra chromosome is made up of copies of arms from two other chromosomes. 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 Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome, with 46.7 million base pairs (the building material of DNA) representing about 1.5 percent of the total DNA in cell ...

, which is found in Down syndrome, is called trisomy 21.

Human trisomy

Trisomies can occur with any

, but often result in spontaneous abortion (miscarriage) rather than live birth. For example, Trisomy 16 is most common in human pregnancies, occurring in more than 1%, but the only surviving embryos are those having some normal cells in addition to the trisomic cells (

mosaic A mosaic () is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/Mortar (masonry), mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and ...

trisomy 16). Furthermore, even these embryos usually suffer

miscarriage Miscarriage, also known in medical terms as a spontaneous abortion, is an end to pregnancy resulting in the loss and expulsion of an embryo or fetus from the womb before it can fetal viability, survive independently. Miscarriage before 6 weeks ...

in the first trimester. The most common types of human

autosomal 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 allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosome ...

(non-sex chromosome) trisomy that survive to birth are: *Trisomy 21 ( Down syndrome) *Trisomy 18 ( Edwards syndrome) *Trisomy 13 ( Patau syndrome) * Trisomy 9 * Trisomy 8 (Warkany syndrome 2) 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,

intellectual disability Intellectual disability (ID), also known as general learning disability (in the United Kingdom), and formerly mental retardation (in the United States), Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010).Archive is a generalized neurodevelopmental ...

and shortened life. ''Trisomy of sex chromosomes'' can also occur and include: *XXX ( Triple X syndrome) *XXY (

Klinefelter syndrome Klinefelter syndrome (KS), also known as 47,XXY, is a chromosome anomaly where a male has an extra X chromosome. These complications commonly include infertility and small, poorly functioning testicles (if present). These symptoms are often n ...

) * XYY (Jacobs syndrome) Compared to trisomy of the autosomal chromosomes, trisomy of the sex chromosomes normally has less severe consequences. Individuals may show few or no symptoms and have a normal life expectancy.

References

{{Authority control Cytogenetics Chromosomal abnormalities

Description and causes

Terminology

Human trisomy

See also

References