Sindromul Down (Trisomia 21) este cauzat de prezenta suplimentara a unui cromozom (si anume  21), persoana cu aceasta afectiune avand 47 de cromozomi in loc de 46. Romania, la nivelul anului 2022, are o incidenta de 1:847 de nasteri, indiferent de rasa, clasa sociala sau localizare geografica. La noi in tara se estimeaza ca circa 12,000 de persoane au aceasta afectiune, iar la nivel mondial sunt peste 6 milioane.

Due to this additional chromosome, all trisomic persons have a degree of retardation in physical and motion activity fields as well as mental and functional development, regardless of the type of trisomy (free/standard (96%), translocation (2%) and inhomogeneous/mosaicism (2%) )). They also have a typical Asian face, with almond-shaped eyes. For this reason, in the past, the condition was called "mongol face/mongol disesease". When they reach maturity, they have, on average, the intelligence of an 8-9 year-old child (but the limits can vary greatly, from individual to individual). These persons can do almost all things that any human can do, but they learn much harder and in a longer time.

In the past, they had a life expectancy of 10-18 years, but now they can easily reach 50-60 years, leading a semi-independent life.