Because I get a lot of emails and mainly a lot of search hits on translocation inherited down syndrome, robertsonian translocation, translocation 21;21, I will start with Lynette’s question:
Where did you find information on this type of translocation?
First of all let me explain what it is: Translocation Trisomy 21;21 is a form of Down syndrome. In this specific case, two 21 chromosomes have grown together and a third 21 chromosome has joined, therefore causing Trisomy 21 (three of chromosome 21) also known as Down syndrome.
What is so unusual about this specific type is that if the mom or dad is a carrier of the two grown together 21st chromosome (so-called balanced carrier), there is a 100% outcome of Down syndrome with each pregnancy. There is no other way. If the egg and sperm fuse, the embryo will have Down syndrome.
Translocation Down syndrome is the form that CAN be, but NOT ALWAYS is inherited.
Translocation 21;21 is very unusual, and it also does not mean that you have to be a carrier. With the people I have come in contact with, almost all cases have been not balanced carrier. The easiest way to find out if you are a carrier is:
a) if you already have a child without DS prior to your DS translocation baby. But this only holds true for Translocation 21;21 (you can also have a DS translocation, 13, 14, 15 and 22, all these are more common that 21;21). By having had a child without DS and then one with T21;21, you can automatically exclude the possibility of you or your partner (assuming the children have the same parents) as if you were a carrier all babies would have had to have DS T21;21.
b) Simple blood test done at the genetics office. Think about this if you decide to do it. Are you fine with finding out the answer? Many individuals choose to do this because of siblings as if you are a balanced carrier there is a great chance/risk that your brothers/sisters also are.
c) There is still a possibility you are a carrier even if your blood test is negative. This is referred to as Gonodal Mosaicism Down syndrome. What it means is that you or your partner is a balanced carrier but only in the sex cells (sperm or egg). I have met only met one family with this type of T21;21. It is very unusual.
It is true, there is not a lot of information out there on translocation. A lot of the useful information I have found I have included in this posts. I will also relist it here again:
1. The 2010 version of Translocation Trisomy 21;21 (incl. karyotype of T21;21)
4. Translocation Trisomy 21/Robertsonian Trisomy 21 (incl. karyotype of T21;21)
7. Finding out Vincent has DS (Bonus entry)