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Chromosomal loss from the early zygote.

Shane McKee shane at reservoir.win-uk.net
Wed Apr 26 10:57:48 EST 1995

Hi. My query relates to the loss of chromosomes from the first few
divisions of the zygote. In Down syndrome, there are three basic
types (with some modifications). Firstly, you have the standard
trisomy 21 (~95% of cases). Second, you have translocations, in
which an extra 21 is stuck to another chromosome (~3%). Third,
there are mosaics, in which some body cells are normal, and some
have trisomy 21.

Now, concerning mosaics, there are two basic ways I can figure of
them coming into being. The first way is that they start out as a
normal zygote, and in one of the early rounds of mitosis, one of
the cell lines gains a Chr21. The second way is to start with
three 21s, and lose one in one of the cell lines.

Work with Prader-Willi & Angelman, among other syndromes, suggests
that in some cases 'extra' chromosomes may be lost during mitosis,
leading to uniparental disomy for some chromosomes, in which both
copies of a chromosome in a zygote come from the same parent.

My question is, has any work been done on the factors which cause
the loss of extra chromosomes during mitosis? I figure that it may
be possible to mitigate the effects of Down Syndrome if
'treatment' to lose an excess 21 were initiated early in pregnancy.

Has anyone any thoughts on the matter?


Shane McKee (JHO, RVH, Belfast)  | /      Art becomes science when
Shane at reservoir.win-uk.net     --O--    you start trying to figure
AGACTGCGCTTGCTTTACACATTTCTTCTC / |  out what the heck you're doing

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