The most common form of Down's syndrome is due to an extra chromosome No. 21, i.e. three instead of two chromosomes 21, Trisomy 21 (T21). This was discovered over 50 years ago. We now know with certainty that the extra chromosome usually comes from the mother (in approximately 19 out of 20 cases) and also that the probability of having a child with Down's syndrome increases with maternal age. However, we still do not know how this comes about.

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