There are three forms of DS - trisomy 21, translocation and mosaicism.
Trisomy 21
This is
the most common form of DS. 95% of children with
DS have an extra whole chromosome in every cell
of their body. It can occur as a result of faulty
distribution of chromosomes in either the mother's
egg or the father's sperm or as a result of faulty
distribution of chromosomes in the first division
of the fertilised egg.
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Translocation
These comprise of about
4% of cases. DS is due to the presence of an extra
part, rather than the whole, of a chromosome .
This occurs when the small top portions of chromosome
21 and another chromosome break off and the two
remaining portions stick to one another at their
exposed ends. Only chromosomes 13, 14, 15 or 22,
or another chromosome 21 (14 is the most common)
are involved. |
