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Edwards' syndrome (trisomy 18)
Normally, people have 46 regular chromosomes (23 pairs) and two sex chromosomes (1 pair). When there is a single extra chromosome (aneuploidy) such that there are 47 instead of 46 chromosomes, the condition is called trisomy. Trisomy 18 (along with trisomy 13 and trisomy 21) is one of the more common aneuploidy conditions. In Edwards' syndrome there are three, rather than the normal two, chromosomes number 18. Children with this condition have multiple malformations and severe mental retardation due to the extra chromosome number 18. Characteristically they have low birth weight, small head (microcephaly), small jaw (micrognathia), malformations of the heart and kidneys, clenched fists with abnormal finger positioning, and malformed feet. The mental retardation is profound with the IQ so low it can not be measured with the usual testing. Nineteen out of 20 (95%) of these children die before their first birthday. The condition is named after the British physician and geneticist, John Edwards, who discovered the extra chromosome in 1960. There is no good prenatal screening test for this condition. It is very uncommon and can be identified on amniocentesis. It is more common with increasing maternal age.
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