[tri-med] Genetics 103 - Recurrence Risks

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!!!!


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

Other related posts: