Medical Genetics

Down syndrome (trisomy 21)

Down syndrome is a chromosomal condition caused by an extra copy of chromosome 21, with variable learning disability, hypotonia and characteristic features plus a higher chance of treatable cardiac, gastrointestinal, endocrine, hearing, visual, sleep, haematological and neurological problems.

Definition

Down syndrome is a chromosomal condition caused by an extra copy of chromosome 21. It is associated with variable learning disability, hypotonia and characteristic features, with an increased chance of congenital heart disease and other treatable health conditions.

First principles

Extra chromosome 21 creates a variable gene-dosage phenotype

Most people with Down syndrome have an extra free chromosome 21 from meiotic nondisjunction; a minority have a Robertsonian translocation or mosaicism. The extra gene dosage affects development across several systems, so phenotype and support needs vary widely between individuals.

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