I think those dogs with that REALLY subtle brindle might actually BE genetically brindle. From what I understand... to get brindle pups, one of the parents has to be brindle. I know a male that has that ''hint'' of brindle in certain lighting although he ''looked'' black if you weren't actually looking that hard.... he was bred to a pied dog, with no brindle and had brindle pups, which in turn means one of the parents had to be brindle. I think anyway!