What Are the Chances of a Person With Down's Syndrome Passing the Condition to Their Children?

Question

I asked this question in another category, but apparently nobody knew the answer or even understood the question.

One form of Down's syndrome is caused by an extra copy of the 21st chromosome. What are the chances of someone with this condition passing Down's syndrome to their offspring?

Answer 1

Robertsonian Translocation Down Syndrome (45,XX, t(14;21q), for example, and this is often referred to as "familial Down Syndr." and it accounts for about 2-3% of cases.

Answer 2

From what I have read, it is rare, but not unheard of for persons diagnosed with downs syndrome to be fertile.  According to wikipedia, there are only 3 reported cases of males with downs syndrome fathering children.  Pretending I didn't know that, I would say it would be about 50% chance with one parent with downs and one without, and considerably  higher with two parents with downs.  About 25% for normal parents with a child previously diagnosed with downs.  

Answer 3

The chances of passing on Down's syndrome are essentially zero.  Down's syndrome is caused by the random failure of a mechanism that prevents extra chromosomes _after_ fertilization.  The gametes produced by someone with Down's syndrome typically fail or are rejected by the body, producing infertility, but on the off chance they produce a gamete that accidentally loses the extra copy of chromosome 21 (which can happen), the gamete would have a _normal_ genome and would not pass on any additional risk of Down's.

Answer 4

The majority of boys with Down Syndrome are sterile, and around half of the girls are infertile. If a young woman with Down Syndrome gives birth , the baby will have a very high chance of also being born with DS.