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Down syndrome
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Down syndrome is the most common viable autosomal trisomy. Trisomy means there are three copies of a chromosome instead of two, and here the extra copy is of chromosome 21. It is the leading genetic cause of intellectual disability.
In about 95% of cases the cause is meiotic nondisjunction: as the egg or the sperm forms, a process called gametogenesis, chromosome 21 fails to separate the way it should. When you deal, one card goes out stuck to another. The remaining cases come from Robertsonian translocations, in which a piece of one chromosome attaches to another, or from mosaicism, when the error happens later and only some of the cells carry it.
One hypothesis is that the extra chromosome adds too many copies of its genes, something called excess gene dosage, and that this disrupts developmental pathways. The signs include characteristic facial features and, in up to half of babies born with the syndrome, congenital heart defects, meaning heart problems present from birth.
In brief
Down syndrome is the most common viable autosomal trisomy and the leading genetic cause of intellectual disability: there is an extra copy of chromosome 21.
About 95% of cases arise from meiotic nondisjunction, when chromosome 21 fails to separate during gametogenesis; the rest come from Robertsonian translocations or mosaicism.
When you deal, one card goes out stuck to another.
One hypothesis is that excess gene dosage disrupts developmental pathways; characteristic facial features are present, along with congenital heart defects in up to half of cases.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingDown Syndrome, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated