Why does incest cause birth defects?

Gene errors: We all carry recessive genes to a few conditions that are not a problem unless they come together with another carrier.The closer you are related, the more genes you have in common. Two carriers, can conceive a baby that has both bad recessive genes, with any pregnancy.Whether this produces a fatal metabolic disorder or some lesser problem, the results are the same. The genes define you.

Consanguinity: Most genetic disorders are autosomal recessive. That means the the defective gene is not able to manifest disease due to the dominant normal gene. In incest the chances for two defective genes to form in one egg is higher due to both members being related and are likely to have similar defective genes.