how syndromes are formed
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The most common form of Down syndrome is known as trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. Trisomy 21 is caused by an error in cell division called nondisjunction. This leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.
The likelihood having Down syndrome is around 1 in every 700 pregnancies. It is determined by many factors, but research suggests there is a higher risk if the mother delivers at over 35 years of age.
Before the age of 30 years, fewer than one in 1,000 pregnancies will be affected by Down syndrome. After the age of 40 years, this figure rises to about 12 in 1,000.
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