Down syndrome
47,XX,+21
Also known as: Trisomy 21, 47 XX +21
47,XX,+21 is the karyotype of Down syndrome, caused by a third copy of chromosome 21. It is the most common autosomal trisomy compatible with live birth.
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What the notation means
47- total chromosome count (normal is 46)
XX- sex chromosomes: two X (usual female karyotype)
+21- an EXTRA copy of chromosome 21 (three copies = trisomy 21)
Clinical notes
Trisomy 21, Down syndrome
The most common autosomal trisomy compatible with life (~1/700 births). Three copies of chromosome 21. Features: characteristic facies, hypotonia, intellectual disability, ~50% congenital heart disease (AV canal), ↑ risk of AML/ALL and early Alzheimer disease. ~95% free trisomy (nondisjunction, ↑ with maternal age), ~4% Robertsonian translocation, ~1% mosaic.