Genetics and genomics of Down syndrome

Abstract Down Syndrome (DS) is the most frequent form of intellectual disability (ID) of genetic origin, whose main features include craniofacial dysmorphisms and cardiovascular defects. In 1959, Lejeune and coll. described an extra copy of chromosome 21 (Hsa21) in children with DS (trisomy 21, or T21). We first review how different biological mechanisms may lead to the gain of genetic material of Hsa21 in the cells, originating from different combinations of genetic conditions, including a free or translocated extra copy of Hsa21, distributed in all cells or only in a part of them (mosaicism), with a complete or partial representation of the Hsa21 long arm (21q). Although it is broadly agreed that the DS phenotype originates from the altered expression of the genes located on Hsa21, its molecular pathogenesis is still unknown. We therefore illustrate how recent genomic science may be useful in the elucidation of the genotype-phenotype relationship in DS.