That's not what the Bible teaches. The promised seed was under attack from Satan, because it would spring from Seth's bloodline. But that only means that some of his descendants were godly, not that the whole bloodline were. Also, Noah was perfect in his generations doesn't have anything to do with corrupted DNA or anything (at least beyond the curse of sin on all creation), but it has to do with his complete and utter devotion to God in a world where humanity had given itself over to evil and great violence.