I don't know for sure in my case, since my DNA has not been sequenced, but missing third molars are, apparently, quite common.
From the Dental Genetics Group website:
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Congenitally missing teeth
Missing of one or more teeth is perhaps our most common congenital malformation. More than 20 % of us lack one or more wisdom teeth (third molars). More than five percent of us lack one or more second premolars or upper second (lateral) incisors. Lack of a large amount of teeth, though, is much more rare.
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