DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
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Down's syndrome. View of a smiling two year old girl affected by Down's syndrome, also known as trisomy 21. Down's syndrome results from a chromosomal abnormality in which sufferers have 47 chromosomes in their cells rather than the usual 46. This is due to an extra chromosome 21, of which there are three instead of the usual two. About one in 650 babies shows this abnormality and there is evidence which suggests that this rate increases with increased maternal age. Down's syndrome results in characteristic facial features, such as eyes which slant up at their outer corners. There is also some degree of mental handicap.
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