David Patterson is a Professor and the Director of External Relations, Eleanor Roosevelt Institute, University of Denver (Colorado), and one of the world's leading researchers in the molecular genetics of Down syndrome. The journal Human Genetics has recently published his excellent review of current research on the 21st chromosome, and current thinking on the complex interactions that are emerging:
Molecular genetic analysis of Down syndrome, Human Genetics. Volume 126, Number 1 / July, 2009 (published online on June 13)
The abstract of the paper and Dr Len Leshin's very useful commentary are available on Dr Leshin's website Down Syndrome: Health Issues. The full text can be read or downloaded as either a .pdf file or in html, from the journal's web page.