In the beginning God created Adam and Eve. They "begat” Cain and Abel. And we were made to know that Abel was killed by his jealous Brother Cain. Cain was latter cast out from his people and he settled down and married. Now the question is where did the woman he cleaved to come from? Was she Adam and Eve's daughter? I don't think so since we were explicitly told that they had only Cain and Abel at the time
I tried to read further to see if I have missed out something in GENESIS. But I was sure I read it well. Eve and Adam "begat" another son after they sent Cain away and after Abel was killed. Their next son was Seth and he married too and "begat" Enoch who married too and bore sons and daughters.
Bearing sons and daughters started from Seth as I understood from my reading... Cain on the other hand "begat" sons who married and "begat" others sons who married too. Where did they get all their wives? Is something missing in Genesis?
Another thing that comes to mind is what colour of skin were Eve and Adam? How where they able to breed white, black, brown, off-white children that now from