Not until the time of Moses were laws established forbidding a man from marrying a sister or niece. The timing of this command makes perfect sense biologically, for genetic defects as a result of intra-family marriage would not begin to crop up until after the first few dozen generations.On the mystery of Cain's mark:
The necessity of this mark indicates that the population of the human race had reached (or would reach) such a level that mistaking Cain for someone else would be a problem.Read the rest of Finding A Wife for Cain on Reasons.org
The text says nothing about what kind of mark Cain received or about its being passed on to his progeny. I see no basis for believing that any one of the races of man carries the mark of Cain.