I agree with Cajun...it is genetic
However, I have 4 pups out of a Pocahontas Plott gyp and the sire is an open 1/2 mt cur 1/4 redbone 1/4 pitbull dog...these 4 pups should be open on track but they have been running silent...the only thing I can figure is that the bark collars they have been wearing at the kennel has caused that so it is possible a dog can be trained to run silent...in the woods they will not have a bark collar but they will be wearing a quick trac and a Garmin collar...
I am thinking they would default back to open if I leave the bark collars off for a long while...
I have wondered about the long term effects of bark collars. Have you noticed any reluctance to bay in the woods or does that hunting instinct overtake the learned behavior of not barking for the dogs?