Trisomy

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Classification according to ICD-10
Q90.- Down syndrom
Q91.- Edwards Syndrome and Patau Syndrome
Q92.- Other trisomies and partial autosomal trisomies, not elsewhere classified
ICD-10 online (WHO version 2019)

A trisomy (from ancient Greek τρία tría , German 'three, dreierlei ' and σῶμα sôma , German 'body' ; here chromosome body as the carrier of the genetic information) is when a chromosome or part of a chromosome is due to an unusual maturation of the egg cell or sperm triple (trisome) instead of double (disome) is present in all or some body cells.

A general distinction is made between simple trisomies (only one chromosome is tripled) and multiple trisomies (several chromosomes are tripled).

root cause

The chromosomes of a person are usually present as a double set and thus in diploid form. Chromosomes can be tripled when cells divide in such a way that instead of one chromosome, two chromosomes of the same number enter the germ cell. The fertilized egg cell then has three chromosomes in total (since there is also one chromosome): One each from mother and father and an additional one from either mother or father. This is known as a genome mutation, which is a special feature of the number of chromosomes. Free trisomies (see below) arise due to non-disjunction during meiosis , mosaic trisomies (see below) arise due to non-disjunction during mitosis .

Any woman of childbearing potential at any age can have a child with some form of trisomy. The likelihood of a trisomy in the child increases with the age of the biological child mother (and possibly also the biological child father), so that in Germany it is mandatory for gynecologists to have pregnant women from the age of 35 as part of prenatal diagnosis for the possibility of an examination of the child Identify chromosomes through procedures such as chorionic villus sampling , amniocentesis, or umbilical cord puncture .

Since trisomies cannot be cured causally and people with tripling of chromosomes usually have physical and cognitive impairments of varying degrees of severity, many pregnant women or pairs of parents decide to terminate the pregnancy .

Forms of trisomy

There are four types of trisomy:

  1. Free trisomy
    This is the most common type of trisomy: The chromosome in question is completely threefold in all body cells. In principle, however, it is possible that a free trisomy may be diagnosed, although a mosaic trisomy (see below) is actually present. Free trisomies arise due to meiotic non-disjunction .
  2. Translocation trisomy
    In this type of trisomy, the respective chromosome is sometimes present in three instead of two in all body cells. An additional part of a third chromosome has attached itself to another chromosome. This “change of location” of a chromosome or a piece of chromosome is referred to in genetics as a translocation , which is an unbalanced translocation because there is more genetic material than usual. The inheritance of a translocated chromosome thus leads to this special form of trisomy.
  3. Mosaic trisomy
    With this type of trisomy, not all cells of the body have a triple chromosome, but rather a cell line with the usual diploid set of chromosomes. The presence of several karyotypes within an organism is called a     mosaic in genetics . Due to the presence of two different cell lines, it can in principle happen that a chromosome analysis only examines samples with a trisome chromosome set, on the basis of which a free trisomy (see above) would then be diagnosed, although a mosaic trisomy is present. In some people with a mosaic trisomy, the characteristics of the respective syndrome are less pronounced, depending on the proportion of disome cells, although this should not be generalized, but always viewed on a case-by-case basis. Mosaic trisomies are caused by mitotic non-disjunction .
  4. Partial trisomy
    This type of trisomy is extremely rare. In people with a partial (partial, partial) trisomy, the chromosomes are present in duplicate in all body cells, as usual, but part of one of the two chromosomes is threefold, which means that one of the chromosomes is slightly longer than the other. The genetic information in this section is therefore available in three ways. Partial trisomy plays an important role in research. It is investigated how the trisomy of certain sections and genes on chromosomes is related to the expression of certain peculiarities, because mostly in people with a partial trisomy characteristics of the respective syndrome are less pronounced depending on the respective trisomal chromosome segment, although this cannot be generalized , but must always be considered in each individual case.

Autosomal trisomies in humans

The best-known form of a trisomy, since it is the most common form in live births worldwide, is the tripling of material on the 21st chromosome. It causes Down syndrome (trisomy 21), which affects an average of 1 in 700 live babies. People with this peculiarity are usually viable, able to learn and work, and their quality of life is good, especially depending on their social environment. This is due to the fact that the 21st chromosome is the smallest in humans and therefore there is less genetic disorder than other trisomies.

Most other trisomies cause serious physical developmental disorders and are either incompatible with life or have a very short life expectancy. As a rule, a miscarriage occurs , often at such an early stage of pregnancy that the loss of the embryo is not noticed as such, but the onset of bleeding is attributed to an irregularity in the menstrual cycle . The only other trisomies besides trisomy 21 that are viable as free (i.e. full) trisomies are trisomy 18 ( Edwards syndrome ) and trisomy 13 ( Patau syndrome ), with both the risk of miscarriage being very high and the average life expectancy very high is low, even if individual sufferers reach adulthood. As partial trisomies or mosaic trisomies, some more trisomies are viable. These include trisomy 3q , trisomy 9p , trisomy 10p and trisomy 12p , trisomy 8 and trisomy 9 .

Gonosomal trisomies

Often and in some cases associated with significantly less physical and cognitive peculiarities are triples of the sex chromosomes X and Y. These include Klinefelter syndrome (XXY syndrome), XYY syndrome and triplo-X syndrome (trisomy X).

Features of people with trisomy

The presence of some form of trisomy usually causes some abnormalities in a person's physical and cognitive development. Often found z. B. Organ malformations and limited cognitive performance. These features appear to different degrees in different trisomies. In general, the intensity of the respective characteristics depends on the development; the earlier the division error occurred, the more often it was "copied" by the genetic duplication and the more strongly the characteristics are mostly represented.

Educational and medical-therapeutic interventions contribute to the fact that the majority of children with some form of trisomy can develop cognitively and physically quite well today. It must always be taken into account that people with a form of trisomy, despite their particular genetic makeup, are individuals who, depending on the respective possibilities of investment and environmental factors, develop individual weaknesses and strengths. More detailed information can be found in the respective articles about individual features that frequently occur in special trisomies:

Trisomy syndromes

See also

literature

  • Klaus Sarimski: Developmental Psychology of Genetic Syndromes . Hogrefe, Göttingen 2003, ISBN 3-8017-1764-X .
  • Regine Witkowski, Otto Prokop , Eva Ullrich: Lexicon of Syndromes and Malformations . Springer, Berlin 2003, ISBN 3-540-44305-3 .

Web links

Wiktionary: Trisomy  - explanations of meanings, word origins, synonyms, translations