Larsson L and Hulten MA
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.
It is generally accepted that, in rare cases, the mother has carried the extra chromosome 21 since the time when her ovaries were developing, when she herself was a fetus. On the other hand, it is also generally accepted that, in the majority of cases, the extra chromosome is due to an error in the so-called reduction division (meiosis), when the number of chromosomes are halved in the formation of egg cells. The reduction division begins when the mother is a fetus and is not finished until her monthly ovulation, from puberty until menopause.
Maj Hulten, Professor of Clinical Genetics, believe in contrast to most other researchers that many women may be low-grade ovarian T21 mosaics. In other words their ovaries contain both cells with the normal chromosome number as well as those with an extra chromosome, trisomy 21. This in turn could explain both the origin of T21 Down's syndrome in children of younger mothers, and the so-called maternal age effect, i.e. that the probability of having a child with T21 Down’s syndrome increases with maternal age.
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