Your basic assumption that DNA is unimportant is wrong.

Basic counter examples are:

Reflexes. The variation in human reaction to drugs. Down syndrome and other genetic defects.

It's the similarity in the basic human genome that lets most people assume we are blank slates. Generic mutation can alter an infant's reaction to their environment and people don't notice X-Ray's due to basic biology. Thus we are not blank slates even if environment seems to dominate the picture.