Diagnosis of syndromes caused by autosomal aberrations
Last reviewed: 23.04.2024
All iLive content is medically reviewed or fact checked to ensure as much factual accuracy as possible.
We have strict sourcing guidelines and only link to reputable media sites, academic research institutions and, whenever possible, medically peer reviewed studies. Note that the numbers in parentheses ([1], [2], etc.) are clickable links to these studies.
If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Karyotyping is the main method for diagnosing these syndromes. It should be noted that methods for identifying chromosomal segmentation accurately identify patients with specific chromosomal abnormalities, even in cases where the clinical manifestations of these anomalies are insignificant and of little specificity. In complex cases, karyotyping can be supplemented by in situ hybridization.
Down's syndrome (trisomy 21, trisomy G, Mongolism) is the most common form of human chromosomal pathology. Approximately 95% of cases of Down's syndrome in the patient have an additional chromosome 21. At the heart of the disease is the absence of divergence of the 21st pair of chromosomes either in the oocyte during meiosis, or in the early stages of fragmentation of the zygote. The patient's karyotype in trisomy contains 47 chromosomes (1 extra chromosome 21). In addition to the classical form of trisomy, chromosome variants are possible.
With the translocation variant, 46 chromosomes are present in the karyotype of the patient, but in reality in this case there is a genetic material of 47 chromosomes - the additional chromosome 21 is translocated. Most often an additional chromosome 21 is attached to the chromosome 14 - t (14; 21). In about half the cases, the parents have a normal karyotype. In the other half of the married couples, one parent (almost always a mother) with a normal phenotype has only 45 chromosomes, one of which carries a translocation t (14; 21). In such a family, the risk (1:10) of the re-birth of a child with Down's syndrome is increased, since in meiosis in a parent with an abnormal karyotype, along with normal gametes, gametes with an unbalanced karyotype will appear.
The next translocation of frequency is t (21; 22). If the translocation is present in a woman, the risk of having a sick child is 1:10, if a man, the risk is negligible. Very rarely, t (21; 21) is observed, in such cases the risk of having Down syndrome in the offspring is 100%.
Another variant of Down's syndrome is mosaic trisomy 21. As a result of the chromosome discrepancy in the zygote, two lines of cells - with a normal karyotype and with 47 chromosomes - are found in some patients. The relative proportion of each cell line can vary both between individuals and in different organs and tissues of the same individual. The risk of the birth of a child with Down syndrome in a carrier of mosaic trisomy 21 is determined by the degree of gonad mosaicism.
Trisomy 18 (Edwards syndrome). An additional chromosome 18 is found in 1 out of 3,000 newborns. The frequency of the syndrome increases with the age of the mother.
Trisomy 13 (Patau syndrome) is a syndrome caused by trisomy 13 and characterized by the presence of multiple developmental defects in newborns (defects of the interatrial and interventricular septum in 80%, digestive organs, polycystic kidneys, myelomeningocele in 50%).
Partial trisomy 22 (feline pupil syndrome) - a syndrome characterized by the presence of an additional acrocentric chromosome 22 (22q +), is manifested by iris coloboma, an atresia of the anus, congenital heart defects, severe retardation of mental development.
[