Each cell of an individual contains all the hereditary material that defines it phenotypically. This material (DNA), is made up of 3×109 base pairs per haploid genome. In the metaphase stage of mitosis and of meiosis I, the DNA is condensed to a maximum in the form of chromosomes.
Cytogenetics is the discipline that studies the number and structure of chromosomes. The karyotype or chromosomal chart is made up of 46 chromosomes: 22 pairs of autosomes and one pair of gonosomes determining the genotypical gender. The autosomes are numbered from 1 to 22, from the longest chromosome (2.5×108 base pairs), chromosome 1, to the smallest (5×107 base pairs), chromosome 21. A chromosomal band contains about 3×103 to 3×105 base pairs; the standard level of chromosomal resolution is 400 to 550 bands. Finer test of chromosomal imbalance can now be performed by DNA chip (ACPA, CGH array, SNP array).
The constitutional karyotype is determined with a whole blood sample using circulating lymphocytes (postnatal karyotype) or on fetal cells from amniotic fluid, or fetal blood (prenatal karyotype).
The chromosomal study enables the detection of imbalanced anomalies (e.g. Down syndrome, monosomy X, deletions, duplications) and other balanced anomalies (e.g. translocations, inversions, insertions, etc.).
Testing for microdeletion or microduplication is performed by fluorescence in situ hybridisation (FISH) for Willi Prader/Angelman 15qq12 syndrome, Di George 22q1 syndrome, Williams Beuren 7q1 syndrome, Cri du chat (5p-) syndrome, Wolf Hirschhorn (4p-) syndrome, Léri-Weill syndrome with SHOX deletion etc.
With suspected mental retardation, or in the case of major echographic antenatal warning signs, the chromosomal study can be complemented with a DNA chip analysis, which has 100 times the resolution of the karyotype, and which can indicate certain uniparental isodysomies.
A karyotype can also be requested in cases of reproductive difficulty; in France, 25% of karyotypes with this indication are carried out in Eurofins Biomnis laboratories.