To marry near of kin in the ancient world was common. Yet, beginning about 1500 B.C., God forbid this practice. The reason is simple - the genetic mutations (resulting from the curse) had a cumulative effect. Though Cain could safely marry his sister because the genetic pool was still relatively pure at that time, by Moses' day the genetic errors had swelled. Today, geneticists confirm that the risk of passing on a genetic abnormality to your child is much greater if you marry a close relative because relatives are more likely to carry the same defective gene. If they procreate, their offspring are more apt to have this defect expressed.

Leviticus 18:6 No one is to approach any close relative to have sexual relations. I am the Lord.

The Bible is a History book, not a Science book, but it is remarkable that where the Bible touches on Science, not only is it correct, but the writer could not have possibly known what he was talking about! How did the writer know these scientific facts? It must have been revealed to him by the Creator of the universe.

The Bible is unlike ANY book ever written. It is, in fact, the Word of God.

Source: Eternal Productions