Do sacral dimples need to be evaluated? please help.?

my 1 year old son was born with a sacral dimple above his anus. they doctors never did an ultrasound or evaluated it because they said i looked fine. its small, not very deep. in fact with now that he is 1 year old he its starting to be less noticeable. his doctors checks his dimple every time he sees him. he also has a mongolian spot (greenish gray spot) witch i know its harmless and very common. he is a very active little boy he walked early. he tries to climb on to the sofa and is always very active. my concern is that i was searching the web and it said sacral dimples can indicate a neural tube defect. im scared. do sacral dimples need to be evaluated like a xray or mri for any underlying problem. my mother in law told me one of my husbands brother has such dimple. can sacral dimples be hereditary?