1 Chronicles 20:
"And yet again there was war at Gath, where was a man of great stature, whose fingers and toes were four and twenty, six on each hand, and six on each foot: and he also was the son of the giant."
6-fingeredness, or hexadactyly, is often "corrected" early on, when detected in babies, by the tying off of the extra finger or toe.
To avoid hexadactyly in the first place, according to medical professionals, one had ought to merely avoid engaging in intimate relations with one's own first cousins. Food for thought.