And on the Mosaicism issue- if the father had the mosiacism thing too with HIS sperm also being normal sperm, then fine, they together would have perfectly healthy children *barring random mutations or such things.
Fine. That beats my Translocation 25% answer.
“Almost certainly” isn’t quite the same as certainly. I know of two families where both parents have Down’s and which do have children. One couple has a single son (with Down’s), the other one has two children (but I don’t know whether they have Down’s, all I know is that they’re being raised by relatives).
Hence, the “almost” qualifier.
The Down syndrome man is almost certainly not the biological father of those children.