Down's syndrome mongolism
Down's syndrome (formerly) mongolisma human abnormality caused by the presence of an extra AUTOSOME (number 21) in the chromosome complement of each cell (see ANEUPLOIDY). The main features of the syndrome are: mental retardation, short stature with stubby fingers, characteristic slanting eyes (hence ‘mongolism’) and heart defects. The abnormality arises in the mother's ovum, as a rule, with older mothers having a far higher chance of producing abnormal ova than younger mothers. This may be because the mother's eggs have been part-way through Prophase 1 of MEIOSIS since birth, so that eggs of older mothers have been at risk of NONDISJUNCTION much longer than those of younger mothers. Down's syndrome is the most common chromosomal abnormality in humans, with an incidence of 1 in 600 live births.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005