Trisomy 13 -- occurring in 1 in ∼20,000 births -- is one of only three thoroughly-studied viable non-mosaic autosomal disorders (the others being trisomies 18 and 21), all of which result in stunted growth, mental retardation and multiple congenital deformities. Trisomy 13 (aka Patau Syndrome) leads to severe growth and mental retardation, usually accompanied by: a sloped forehead; malformed ears; clenched fists and rocker-bottom feet (as in trisomy 18); congenital heart defects; urogenital defects; and polycystic kidneys. Most trisomy 13 individuals gain their extra chromosome via maternal nondisjunction in meiosis I.
Many thanks to ThereseAnn, the commenter who provided incredible links to learn more about trisomy 13. I urge StudentReader.com visitors to peruse LivingWithTrisomy13.org, a site about fellow human beings living with Trisomy 13.