Pathogenesis (development of disease)
Chromosomes are strands of deoxyribonucleic acid (DNA) with what are called histones (basic proteins inside a nucleus) and other proteins; the mixture of DNA, histones, and other proteins is also called chromatin. They contain genes and their specific genetic information.
In the human body, the number of chromosomes is 46, with 23 inherited from each parent. Chromosomes are divided into autosomes and gonosomes (sex chromosomes). The chromosome pairs 1 to 22 are autosomes, the 23rd chromosome pair are the sex chromosomes (XY in male and XX in female).
More than 95% of Down syndrome is caused by a triplication of chromosome 21, in this case it is called free trisomy 21.
In so-called translocation trisomy 21, which accounts for 3-4% of Down syndrome cases, chromosome 21 is also present in triplicate, but the additional chromosome 21 has attached to another chromosome.
Mosaic trisomy 21 is observed in about 1% of cases, with triple presence of chromosome 21 only in some cells.
The last group, Down syndrome, unspecified, includes various rarities. Among them is partial trisomy 21, which has been observed only in about 200 cases and has as a specific feature the double presence of chromosome 21, of which one set of chromosomes is longer pronounced and in some places the respective genome section (hereditary segment) is duplicated.
Etiology (causes)
Biographic causes
- Genetic burden
- Genetic risk:
- Meiotic non-disjunction (mis-segregation in meiosis) as a trigger of free trisomy 21.
- Translocation (chromosome displacement) as a trigger of translocation trisomy 21.
- Mitotic non-disjunction (mis-segregation in mitosis) as a trigger of mosaic trisomy 21.
- Genetic risk:
