Well, let's go back to the beginning. Besides Cain, there was Seth. Then, the flood came and whisked everyone away except for Noah, his wife, his sons, and his daughters-in-law. Noah surely would have been of the line of Seth. Most likely, his family members would also have been descendants of Seth. Even his daughters-in-law, though we don't know for sure.

Shem was one of Noah's three sons. It is thought that Ham was black, Shem was Semitic, and Japheth was light-skinned. The word Semitic comes from the name Shem.

The most important thing to remember is not so much the DNA of a person but the SPIRIT of a person. Even if every single descendant of Cain was taken away in the flood, the spirit of Cain (being all about one's self) can arise in anyone.

Example: Gideon was a very righteous man whom God chose to war against the Midianites with 300 men. After Gideon died, his son, Abimelech, was EVIL. Even within a nuclear family, there can be good and evil. Each person chooses.