Is human quadrupedalism due directly to genetic factors?

This paper that I blogged about a couple months ago about the possible genetic basis for quadrupedalism (walking on all fours) among a family in Turkey finally came out in PNAS and elicited a couple of letters in the same issue of PNAS (here, here) criticizing the interpretation of what they find. They bring up families in Iraq and among the Hutterites who are quadrupedal yet don't have the same mutation, or have the same mutation but aren't quadrupedal. Both of these letters suggest that quadrupedalism is not due directly to genetic factors but is "epiphenomenal" in that it's a way of compensating for balance problems caused by cerebellar hypoplasia, and due to a lack of "supportive therapy".