trisomy 21
Trisomy 21 is the specific chromosomal arrangement of having three copies of chromosome 21 instead of the usual two. It is the underlying cause of Down syndrome in the great majority of cases. If Down syndrome is the story, trisomy 21 is the precise typo on a single page that set the story in motion.
It most often arises from nondisjunction during meiosis, usually in the egg, where the two copies of chromosome 21 fail to separate so the egg carries both. After fertilization the embryo has three copies in every cell. Because chromosome 21 is the smallest human autosome and gene-poor, this is one of the few full trisomies compatible with survival to birth.
Notation matters in the clinic: a standard karyotype of free trisomy 21 is written 47,XX,+21 or 47,XY,+21, signalling 47 total chromosomes with an extra 21. Translocation Down syndrome instead shows 46 chromosomes with one fused to another (for example 46,XX,der(14;21)), which has different implications for inheritance and recurrence risk.
Noninvasive prenatal screening estimates the chance of trisomy 21 by counting chromosome-21 DNA fragments in the mother's blood; a positive result is then confirmed by a diagnostic karyotype.
Screening flags the chance; a karyotype confirms the actual chromosome count.
Trisomy 21 is a chromosome finding; Down syndrome is the clinical condition it usually produces. Most cases are free trisomy, but translocation and mosaic forms also exist.