[tri-med] : Genetics 103 - Recurrence Risks

MeMe, grandmother to BeBe:

Here's some info that has been posted before by list Mum Karen on the
subject you had questions about.

Welcome to the group!

Michelle mom to Alex (17, partial trisomy 14 mosaic) and Molly (14)
MichiganUSA

> This is a cut and paste of two posts - the first part refers to the risk
of
> recurrence in Trisomy 18 (full) and the second part the risks associated
> with translocations. I will send a separate post with more detailed
> information about risk based on age.
>
> > Thanks Paula.  I am wondering if T18 is such a freak thing, then why do
> our
> > chances increase so greatly of it happening again.  Do they think if one
> > egg/sperm was affected that their is a higher chance of others being
> affected
> > also?
>
> In general the chances of a trisomy occurring again are less than 1% over
> the rest of the population based on age and excluding translocation. So I
> am
> not sure what you mean sweet by increasing so greatly???
>
> In general the risk of any problem happening during a pregnancy for any
> person is 2% - 3%, that is 2 to 3 out of every hundred pregnancies will
> result in some sort of congenital abnormality eg spina bifida, chromosomal
> abnormality, club foot, heart defect etc etc etc.
>
> Once you have had a child with a chromosomal abnormality (that is not
> related to a balanced translocation!!) that risk only increases by less
> than
> 1%. But like everyone else you still have that other 2 -3% chance. I guess
> thats what seems so unfair about this whole funpark!!! I think that
dealing
> with Trisomy is enough - why do we need all the other "normal" worries as
> well!!!!!!
>
> The risk based on age however increases far more rapidly. Now why that is
> again no-one is really sure. What we do know is that the age related curve
> for Trisomy is a three hump curve :-)
> There is a small increase (quite small) for chromsomal abnormalities in
> teens, then it sort of levels out until age 35 where it climbs again
fairly
> sharply.
> As I said why they don't know. Is there something in hormone production
> during the "peak" reproductive years that helps a healthy mother "get rid
> off" "abnormal" embryos through spontaneous miscarriage? Is this the age
> that "healthy" eggs are released and the less "perfect" ones are left to
> later?? Is it because the mother is healthy that some are able to carry
> these babies to term?? Sorry no-one really has the answers as yet and
> because of the nature of the research that would be needed to find out we
> are unlikely to get any real answers.
>
> From: <pjvictor@xxxxxxxxxxxxx>
> > She told me that if Nicholas were a carrier that his
> > chances of having a child (ie: miscarriage,
> > stillborn, trisomy 13, or another carrier) would
> > fall in the 10% catagory. I asked why this figure
> > was so significantly less
>
> OK Penny and Joe,
> I thought I knew this answer and then Penny's answer threw me soooooo I
> asked a genetic counsellor :-))
>
> Here goes!!!
> There are two types of translocations - reciprocal and Robertsonian.
> Now given the situation where this is an existing family condition so that
> there is no mosaicism involved then every sperm would carry the
> translocation in both cases.
>
> So in the case of a reciprocal translocation there are 4 possibilities for
> a
> fertilised egg
> Normal chromosomes
> Balanced translocation
> Unbalanced plus some extra chromosome
> Unbalanced minus some chromosome
>
> so the theoretical chance (and I emphasise theoretical!!!! - read to the
> end) is a 50/50 chance of there being something wrong.
>
> Now in a Robertsonian translocation there are six possibilities (I will
use
> the example of an 18/13 translocation)
> Normal
> Balanced
> Unbalanced with an extra 18
> Unbalanced minus an 18
> Unbalanced with an extra 13
> Unbalanced minus a 13
>
> So theoretically the risks are higher however a monosomy 13 and a monosomy
> 18 are "incompatible with life" and will result in a miscarriage (possibly
> just a "late" period). So the chances are 2 to 1 of something going wrong.
>
> HOWEVER
> every situation is different!!!! and anyone with a translocation should go
> armed with their karyotype and talk specifics with a genetic counsellor.
> This is a very brief THEORETICAL overview!!!! (I spent an hour listening
to
> the variations!!! chances of partials, "normal" risks added in etc etc.)
>
> The other be wary thing is that males who are carriers of a translocation
> have babies with problems less often than women but they HAVE NO IDEA
> WHY!!!
> Its an observation of the population not something that can be explained
by
> science. (But can science ever explain anything fully?????)
>
> Now one other titbit of information is that because sperm are totally
> replaced every three months or so age continues to play no factor in
> increased risk with males. They use the same template all their lives and
> unless there is an environmental effect on that template they will produce
> "mistakes" at the same rate all their lives.
> It is only women who suffer the increased risks due to age and the
> splitting
> of the egg at ovulation.
> So sperm and eggs make the same number of "mistakes" up until age 35 when
> the risk increases for the eggs but not the sperm.
>
> I hope that helps :-)) and doesn't scare anyone, remember this is all
> theory
> we all know that reality is very different!!!!
>
> Take Care and Keep Looking for Rainbows!!!
> Karen, mum to Alex ( T-18 mosaic)


                  Building ___ooOOoo__ Rainbows
                       www.trisomyonline.org
                  Families Helping Families On-line

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