Today, brothers and sisters (and half-brothers and half-sisters, etc.) are not permitted by law to marry because their children have an unacceptably high risk of being deformed. The more closely the parents are related, the more likely it is that any offspring will be deformed.
There is a very sound genetic reason for such laws that is easy to understand. Every person has two sets of genes, there being some 130,000 pairs that specify how a person is put together and functions [Ed. note: This was an estimate from the number of different proteins. But after the Answers Book was published, the Human Genome Project discovered that there are only about 35,000 genes. This is an additional layer of complexity, since these genes must still somehow produce all the proteins. See Genome Mania—deciphering the human genome: what does it mean?] Each person inherits one gene of each pair from each parent. Unfortunately, genes today contain many mistakes (because of sin and the Curse), and these mistakes show up in a variety of ways. For instance, some people let their hair grow over their ears to hide the fact that one ear is lower than the other—or perhaps someone’s nose is not quite in the middle of his or her face, or someone’s jaw is a little out of shape—and so on. Let’s face it, the main reason we call each other normal is because of our common agreement to do so!
The more distantly related parents are, the more likely it is that they will have different mistakes in their genes. Children, inheriting one set of genes from each parent, are likely to end up with pairs of genes containing a maximum of one bad gene in each pair. The good gene tends to override the bad so that a deformity (a serious one, anyway) does not occur. Instead of having totally deformed ears, for instance, a person may only have crooked ones! (Overall, though, the human race is slowly degenerating as mistakes accumulate, generation after generation.)
However, the more closely related two people are, the more likely it is that they will have similar mistakes in their genes, since these have been inherited from the same parents. Therefore, a brother and a sister are more likely to have similar mistakes in their genes. A child of a union between such siblings could inherit the same bad gene on the same gene pair from both, resulting in two bad copies of the gene and serious defects.
However, Adam and Eve did not have accumulated genetic mistakes. When the first two people were created, they were physically perfect. Everything God made was ‘very good’ (Genesis 1:31), so their genes were perfect—no mistakes! But, when sin entered the world (because of Adam—Genesis 3:6ff, Romans 5:12), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over thousands of years, this degeneration has produced all sorts of genetic mistakes in living things.
Cain was in the first generation of children ever born. He (as well as his brothers and sisters) would have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with (it takes time for these copying errors to accumulate). In that situation, brother and sister could have married with God’s approval, without any potential to produce deformed offspring.
By the time of Moses (a few thousand years later), degenerative mistakes would have built up in the human race to such an extent that it was necessary for God to forbid brother-sister (and close relative) marriage (Leviticus 18–20).12 (Also, there were plenty of people on the Earth by now, and there was no reason for close relations to marry.)
