Hi Open,
After reading this article I had to do a little research. What you say here about down syndrome people being infertile, The National Advocate for People with Down Syndrome Since 1979,
Are Males with Down Syndrome Fertile?

Scientific information about the fertility of men with Down syndrome is limited. There have been at least three documented cases where the paternity of a man with Down syndrome was confirmed. It is likely that additional cases will be recognized - especially since more men with Down syndrome have an increased life expectancy, have the opportunity to live in the community, receive treatment for physical and sensory impairments, receive optimum nutrition, and develop intimate relationships. It is not known if the offspring of men with Down syndrome are more likely to have Down syndrome.

It does seem clear that, in general, men with Down syndrome have a significantly lower overall fertility rate than that of other men of comparable ages. An individual's status can be partially assessed by having a semen analysis (sperm count) done, but this may not be definitive. Contraception should always be used, unless a couple has decided upon parenthood.

This was the first of many articles I have found that says, DS people are fertile.
I do not look for reasons to explain things much any more and believe that there are more things, I will not understand, than things I will understand and that is OK too.