If we look at child deformities they are occurring more and more often, as the species expand the genetic chromosomes are becoming more and more spread out so if we have a child with our sister/brothers of cousins the baby is plagued by deformities. If we look back to genies Cain married and had children with one of his sister had that went on for many years the reason that there children did not have deformities is because of the vast amount of genetic traits God put into Adam and Eve now all the information that was in those 2 people are spread out over Billions. So I ask you, will this become problem in the foreseeable future?
If Cain existed, then we would have to have evidence that God protected his children from having deformities. If Cain existed, we would have to have evidence backing any idea as to why his children came out without any deformities.
However, you need to first prove that Cain existed. After you prove that, then you must prove that God protected him.
God can be used as an answer for everything, which means you simply can not lose... unless I lock this thread, in which case, nobody wins except for me.
If you want to talk about God, go to your thread, Debate: Jesus is Lord.
In all seriousness, you're thinking the opposite way. You get common deformities and mental retardation when close relatives mate. When you mate with a distant stranger that was part of a different family line, your children will be mostly fine.
But we don't know. The genes are unpredictable. This is something relatively new to us, so we don't know why genes produce deformities. We just know that there are genes that don't get paired correctly or if they mutate to what is unfavorable.