Genetics. Named after John L.H. Down, 1828-1896. A human congenital disorder caused by trisomy of chromosome 21, occurring in about one out of 1000 births and associated with advanced maternal age. It is characterized by slow physical development, mental retardation, and a number of physical abnormalities, including mongoloid facial features. It is often accompanied by heart disease, vision defects, chronic respiratory infections, and other disorders.