On the average, each human has about six recessive alleles that would be lethal if expressed.?

Question

Why do you think that human cultures have laws against marriage between close relatives?

Answer 1

Relatives are more likely to have the same recessive alleles, so their offspring are more likely than the rest of the population to get two copies of the lethal recessive alleles. However, this does not mean that their offspring would be doomed. It's just more likely that they would inherit these lethal genes.

Answer 2

what he said, but it must be noted that there is a lot of of these alleles, so that its very unlikely that any non-relation will have any of the same six as you.