mosaic Down syndrome

n. A congenital condition characterized by mild to severe mental retardation, pleasant disposition, a flat face, stubby fingers and toes, epicanthic folds in the eyes among nonAsians, tongue fissures, and unusual patterns of skinfolds on the palms of the hands and soles of the feet. It is caused by the presence of an extra chromosome 21 in some but not all of the body's cells. This occurs when there is an error in replicating chromosome 21 at some early stage of development so that an extra copy of the chromosome is produced and subsequently will be present in whatever portion of future cells develop from the original aberrant one.