I was wondering what are the biochemical processes that cause down syndrome people to be more prone to congenital heart diseases?

Unknown: The heart is a very complex organ. We still do not fully understand how the normal heart develops in the fetus. We understand even less about what causes the heart to develop abnormally. Although 40-50% of patients with Down syndrome have congenital heart dis., and we know what causes down syndrome, we do not yet understand why this leads to heart defects (likely due to abnormal gene expression).