Autosomal chromosomes

Down's syndrome is caused by one of two conditions, trisomy 21 or translocation of chromosome 21. It is the most common specific cause of LD, occurring in 1 per 650 pregnancies.

Trisomy 21 does not generally run in families, but becomes more frequent with increasing maternal age, as shown in the following table:

Maternal age

Frequency of trisomy 21 (per 1000 births)

under 35






over 45


Translocation of chromosome 21, which is not related to maternal age, may result from mosaicism in either parent, in which case there is a likelihood of future pregnancies also being affected. This disorder also occurs sporadically.

LD, often severe, is always present. Physical features include a typical facial appearance with slanting palpebral fissures, short little finger with only one palmar crease ('trident hand' and 'simian crease'), hypotonic muscles, cardiac defects (10 per cent of patients), and high susceptibility to chest infections, lymphatic leukaemia, and premature ageing. Patients used to die young, but many now live to middle or old age, often developing Alzheimer's disease in their 40s (an observation which helped to locate an Alzheimer's gene to chromosome 21).

There are differing opinions as to whether all pregnant women should be offered antenatal screening for detection of Down's syndrome; certainly, older mothers, and those with a previously affected baby, should have the option to be screened.

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