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Cytogenetics

Cytogenetics is the study of human chromosomes (number, shape, structure, and
function).

There are normally 46 chromosomes (22 pairs of autosomes and the sex
chromosomes) in a person’s nucleated body cells. This results in the karyotype "46,
XY" for a man and the karyotype "46, XX" for a woman.

Classical Cytogenetics

Conventional karyotyping can detect numerical and structural changes (e.g. deletions, duplications, inversions, translocations) of chromosomes from 5-10 megabases. For this purpose, nucleated, divisible cells are stained using the Giemsa banding method (GTG staining) and the band pattern characteristic of each chromosome is evaluated under a light microscope. The karyotypes are described according to the internationally valid nomenclature (ISCN 2016).

Classical cytogenetics is used both prenatally and postnatally. Amniotic cells, chorionic villi or umbilical cord blood are used for the prenatal examination, while heparinised blood is usually used for postnatal analysis.

Molecular Cytogenetics

Fluorescence in situ hybridisation (FISH) represents a connection between the classical cytogenetic and the molecular genetic approaches, in which fluorescence- labelled DNA probes are hybridised to the respective complementary chromosome segments and subsequently analysed using fluorescence microscopy. The hybridisation is done in-situ, i.e. directly on the chromosomes of the patients.

In addition to the classic FISH analysis, which usually takes 2 weeks, the FISH examination is mainly used in our prenatal FISH rapid test. A preliminary statement can be made about the ploidy level of chromosomes 13, 18, 21, as well as sex chromosomes and known microdeletion syndromes (including DiGeorge) within 24 hours to a few days. In addition, special FISH examinations for the detection of inversions and translocations are also offered in our laboratory.

Array CGH analysis is a high-resolution genome-wide analysis for the detection of gains or losses in the genome. In this process, the DNA is hybridised on millions of fluorescence-labelled probes, which are attached to a chip, and analysed in a scanner. This examination not only detects numerical and structural chromosomal defects, but also the well-known microdeletion syndromes. However, small deletions or duplications or repeat expansions cannot be examined with this analysis; this clarification is carried out with the help of Sanger sequencing or fragment analysis. Identified gains or losses can be confirmed by using MLPA or qPCR.

Array CGH analysis is primarily used by us when the prenatal classical chromosome analysis has yielded an inconspicuous result or postnatally to clarify syndromic  abnormalities. In addition, the analysis can be used in the context of miscarriage diagnostics if no cells capable of division are available and a classic chromosome analysis can therefore not be carried out.