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Re: Trisomy 9From: Allen Gardner (allen.gardner@utoronto.ca)Wed Jun 12 20:25:05 1996
The problem with mosaicism is that you don't know whether you started with a trisomic pregnancy and then lost a 9, or started with a disomic pregnancy and gained a 9. The former scenario is the one that has been caused by meiotic nondisjunction i.e. the one potentially increasing the risk. As far as I know, the only comprehensive and reliable data comes from +21 and most everything else is extrapolated from that. There seems to be 2 schools of thought: 1) one may have an inherent increased susceptibility to meiotic nondisjunction involving any chromosome (only a few of which are viable) 2) If it's +21 then 21 is at increased risk and no others and a different trisomy in a PoC does not raise the risk for +21 in a future pregnancy. Dorothy Warburton's study supports this. The implication is that in 1) finding a trisomy in a PoC becomes an indication for amnio whereas in 2) it does not (unless it's +21) So, if we go back to the original +9 mos question, the worst scenario is a meiotic ND ---> +9 with mitotic anaphase lag and loss of the 9 ---> mosaicism and a belief in 1) above: that meiotic ND is an inherent tendency in this couple with a consequent risk that next time it may be +21. Amnio is indicated. Best scenario: the zygote was diploid, mitotic ND---> +9 mos. Amnio is not indicated. What do you do? I would offer amnio. I'm also a believer that any trisomy (found in LB or PoC) increases the risk for any other trisomy. Sorry to take so much space, but I sure would like to hear how others approach this. Allen.
-- Allen Gardner MD Genetics Oshawa General Hospital Oshawa, Ontario, Canada 905 433 2733
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