Is Down's Syndrome hereditary?
Many "siblings" of people
with DS often ask themselves if they are more likely to
have children with DS than those who don't have people with
DS in their family. To answer this question it's better
to know a few general medical facts. There are three types
of DS. The first form is trisomy 21 due to "nondisjunction"
and accounts for about 95% of all cases of DS. In this case
the extra chromosome is present in the sperm or in the egg,
or in the first cellular division and, as a consequence,
each cell that forms will have three chromosomes in the
couple 21. This form is strictly linked to maternal age
not to hereditary genetic factors. The second form of DS,
which is also the least common, is the mosaicism. This represents
about 2% of cases. In this form of trisomy 21 the cellular
division occurs in a defective way after fertilisation,
during the second or third division, or those following.
As a consequence, not all the cells of the embryo that is
forming contain an extra chromosome. This form also does
not depend on hereditary factors. The third form, which
is very rare, is partial trisomy 21 due to Robertsonian
Translocation, accounts for 2-3% of cases. In this case
a part of 21st chromosome breaks up during the cellular
division, "moves" and attaches itself to another chromosome,
usually the 14th chromosome or the other 21st chromosome.
Some extra genetic material is present in both cases. About
2/3rds of translocation occur spontaneously during fertilisation.
The translocation is hereditary from one of the two parents
in 1/3rd of cases. In this case we have the only form of
DS which is linked to a genetic factor from either the father
or the mother. The parent who is the carrier of this is
normal, but two of his/her chromosomes are united in a way
that the total number of his/her chromosomes is only 45
instead of 46. It is important to identify, through the
child's karyotype the presence or not of partial trisomy
21 due to translocation because if one of the parents is
a carrier the chances that other children with DS can be
born into that family are higher. So, it is advisable that
the parents, the other children without DS and the blood
relatives of the carrier parent have their chromosomes mapped.
So, in conclusion, we can say that the chances of a brother
or sister of someone with DS having children with DS are
only minutely higher than those with no case of DS in their
family, and concern only the form of translocation. Therefore,
it is important to determine whether the brother/sister
has got this form and, in this case, whether they are healthy
carrier. The chromosome map or "karyotype" can be carried
out on a simple blood test. Together with the blood test
you can ask for a "genetic consultancy", that is a more
precise and personalised explanation.