Phenotypic Heterogeneity and Parental Origin of Extra Chromosome 21 in Down Syndrome.

We compared the frequency of phenotypic features of 40 children with Down syndrome between individuals with a maternally or paternally derived extra chromosome 21, using quantitative FISH for comparing heteromorphisms of the nucleolar organizing region. Parental origin was determined in 90% of families. Hypotonia and craniofacial abnormalities were present in 90% or more individuals, irrespective of parental origin of chromosome 21. Congenital heart defects were more frequent in cases with a maternally derived extra chromosome 21. Imprinted gene(s) may contribute to the development of congenital heart defects in Down syndrome.