Every cell in an organism contains all the genetic material that determines its phenotype. This material is DNA, which consists of 3,109 base pairs per haploid genome. During metaphase, the stage of mitosis and meiosis I at which chromosomes are formed, DNA condenses to its maximum extent.
Cytogenetics is the discipline that studies the number and structure of chromosomes. A karyotype, or chromosome map, comprises 46 chromosomes: 22 pairs of autosomes and one pair of gonosomes that determine sex. Autosomes are numbered from 1 to 22, from the longest chromosome (2.5 x 10⁸ base pairs) to the shortest (5 x 10⁷ base pairs). A chromosome band contains between 3.103 and 3.105 base pairs, and the standard resolution level for chromosomes is 400–550 bands. DNA chips (ACPA, CGH array, SNP array) can now be used to search for finer imbalances in the genome.
Eurofins Biomnis.
Pre- and Postnatal Constitutional Cytogenetics
A constitutional karyotype is performed using a blood sample containing circulating lymphocytes (for a postnatal karyotype) or foetal cells from amniotic fluid, chorionic villi, or foetal blood (for a prenatal karyotype).
This analysis can detect both balanced and unbalanced chromosomal abnormalities, such as translocations, inversions, insertions, deletions and duplications.
Microdeletions and microduplications can be detected using fluorescence in situ hybridisation (FISH) for conditions such as Prader–Willi/Angelman syndrome (15q11-q13), DiGeorge syndrome (22q11.2), Williams–Beuren syndrome (7q11.23), Cri-du-chat syndrome (5p-) and Wolf–Hirschhorn syndrome (4p-).
In cases of mental retardation or if major ultrasound signs are detected during prenatal screening, the chromosome study may be supplemented by DNA microarray analysis. This is 100 times more accurate than karyotyping and can detect certain uniparental disomies.
Karyotyping may also be requested in cases of reproductive failure. In France, 25% of karyotypes requested for this purpose are performed at the Eurofins Biomnis laboratory.