[Genetic etiology of trisomy]

Trisomies are genetic anomalies marked by the abnormal presence of an extra chromosome in one or more pairs. The most common and most studied form of trisomy in the human species is trisomy 21, or Down's syndrome, because of its prevalence and its medical, psychological and social implications. Trisomy 13 (Patau syndrome) and trisomy 18 (Edwards syndrome) are also fairly frequent and widely considered because of their viability. Each of these forms has varied and specific clinical consequences, often associated with congenital malformations and different levels of intellectual impairment.