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Diagnosis of syndromes due to autosomal aberrations
Medical expert of the article
Last reviewed: 05.07.2025
Karyotyping is the main method for diagnosing these syndromes. It should be noted that chromosome segmentation detection methods accurately identify patients with specific chromosomal abnormalities, even in cases where the clinical manifestations of these abnormalities are minor and non-specific. In complex cases, karyotyping can be supplemented by in situ hybridization.
Down syndrome (trisomy 21, trisomy G, mongolism) is the most common form of human chromosomal pathology. In approximately 95% of cases of Down syndrome, the patient is found to have an additional chromosome 21. The disease is based on the absence of divergence of the 21st pair of chromosomes either in the egg during meiosis or in the early stages of zygote division. The karyotype of a patient with trisomy contains 47 chromosomes (1 extra chromosome 21). In addition to the classic form of trisomy, chromosomal variants are possible.
In the translocation variant, the patient's karyotype contains 46 chromosomes, but in reality, in this case, the genetic material of 47 chromosomes is present - the additional chromosome 21 is translocated. Most often, the additional chromosome 21 is attached to chromosome 14 - t(14; 21). In approximately half of the cases, the parents have a normal karyotype. In the other half of married couples, one of the parents (almost always the mother) with a normal phenotype has only 45 chromosomes, one of which carries the translocation t(14; 21). In such a family, the risk (1:10) of having a child with Down syndrome again is increased, since in the meiosis of the parent with an abnormal karyotype, along with normal gametes, gametes with an unbalanced karyotype will arise.
The next most common translocation is t(21; 22). If a woman has the translocation, the risk of having a sick child is 1:10; if a man has the translocation, the risk is insignificant. t(21; 21) is very rare, in which case the risk of Down syndrome in the offspring is 100%.
Another variant of Down syndrome is mosaic trisomy 21. As a result of the abnormal chromosomal divergence, some patients have two cell lines in the zygote - one with a normal karyotype and one with 47 chromosomes. 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 having a child with Down syndrome in a carrier of mosaic trisomy 21 is determined by the degree of gonadal mosaicism.
Trisomy 18 (Edwards syndrome). An extra chromosome 18 is found in 1 in 3,000 newborns. The frequency of the syndrome increases with maternal age.
Trisomy 13 (Patau syndrome) is a syndrome caused by trisomy 13 and characterized by the presence of multiple developmental defects in newborns (atrial and ventricular septal defects in 80%, digestive organs, polycystic kidney disease, myelomeningocele in 50%).
Partial trisomy 22 (cat pupil syndrome) is a syndrome characterized by the presence of an additional acrocentric chromosome 22 (22q+), manifested by coloboma of the iris, anal atresia, congenital heart defects, and severe mental retardation.
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