Karyotype (Trisomy 21, 13, 18)

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In this article

  • What is the karyotype?
  • Esame of the karyotype
  • Trisomy 21, 18 and 13

What is the karyotype?

The karyotype is the chromosomal inheritance of an individual, that is, the set of his chromosomes. Inside the chromosomes are contained i genes have been identified., which represents the "memory" of all the information that each of our cells needs to do their job correctly.

In the human species, the normal karyotype is of 46 chromosomes, two of which determine the sex of the person: two X chromosomes in females, one X and one Y in males. The others are equal to two by two in terms of dimensions and functions and are identified with a progressive numbering from 1 to 23.

Karyotype abnormality

When the number or structure of chromosomes deviates from the standard model we speak of anomalies of the karyotype, chromosomal malformations that determine conditions of physical and / or cognitive handicap in the unborn child.

The most frequent karyotype anomalies are the trisomie, in which, instead of a pair, there are three identical chromosomes: the best known is Down's Syndrome, or trisomy 21. Others are those of chromosomes 13 (Syndrome in Patau) e 18 (Edwards syndrome). Then there are the monosomy, with only one chromosome instead of two: for example the Turner syndrome, in which there is only one X. These syndromes usually accompany the presence of fetal malformations and may be associated with mental retardation.

The presence of these anomalies can be detected in the prenatal diagnosis through a special exam.

Read also: The dictionary of pregnancy

Esame of the karyotype

The examination of the karyotype (or chromosome map) is performed by analyzing a sample of cells taken through amniocentesis or CVS operations and allows to view chromosomes under the microscope to observe its structure and the 23 pairs. This examination, as already anticipated, allows to identify any anomalies:

  • Numerical anomalies: they occur in the presence of a number of chromosomes other than 46. The aforementioned trisomies and monosomies (such as Turner Syndrome) belong to this category.
  • Structural anomalies: occur in the presence of chromosomal malformations such as the loss of a fragment (dejection), the detachment of a fragment of chromosome that fits again in the same place, but rotated 180 degrees (reversal), the doubling of a chromosome fragment (duplication) or the exchange of a genetic fragment between two different chromosomes (translocation)

While the numerical anomalies always have serious consequences on the individual, in some cases the structural anomalies can manifest themselves with less serious effects. Balanced translocation, for example, has as its only consequence the difficulty of procreation.

When does the exam take place?

Karyotype examination can be done at different times in an individual's life

  • After birth is prescribed in the case of suspected of a chromosomal condition at risk.
  • In adulthood it is normally performed when faced with problems in procreation (infertility or continuous abortions)

In prenatal period the test is done to identify any chromosomal abnormalities in the fetus.

The times for get the outcome they vary from 2-3 weeks (prenatal setting) to 2-3 months.

Trisomy 21, 18 and 13

La trisomy it is a genetic condition in which an individual's cells have an extra chromosome. There are various types of trisomies, which are distinguished on the basis of the chromosome number which features the triplet instead of the pair. Trisomy 21, for example, is so called because the anomaly occurs on chromosome 21.

The most common types of trisomy are:

  • Trisomy 21 or Down Syndrome: the best known condition. It involves cognitive delays of different entities, characteristic somatic traits and motor difficulties of different entities. Some individuals manage to live for several decades.
  • Trisomy 18 or Edward's Syndrome:most of the children who suffer from it are born dead or die within a few days. In the very few cases where the individual survives more than a few years, the physical consequences translate into skeletal and cardiovascular abnormalities, breathing difficulties and cognitive handicaps.
  • Trisomy 13 or Patau Syndrome: when it does not lead to the immediate death of the newborn, this chromosomal condition involves physical malformations (particularly evident in the head, often undersized and in the feet) and severe cognitive deficits. Cases of babies surviving beyond one year of birth are very rare.

FONTI: National Human Genome Research Institute; Nature Education; MedlinePlus

Questions and answers

What is the karyotype examination used for?

Carotype examination is used to identify any chromosomal abnormalities such as trisomies.

When is the karyotype examination done?

The karyotype examination is performed when chromosomal abnormalities are suspected in the unborn child or, in adulthood, to ascertain the causes behind a reproductive difficulty.

What is the karyotype?

It is the set of chromosomes present in the cell of an individual.

- Updated on 16.09.2022

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  • genetic diseases
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  • dictionary of pregnancy
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