What is your source for this ?
Compare and contrast it with other birth defects and the abortion your side of the supreme court endorses.
It has been commonly know for centuries. We've later learned that sexual procreation involves a matching of genes into a double helix, and that people have many flawed jeans that don't work as the should. When the bonds takes place, the working gene dominates. But when people are closely related, they share the same non-working genes and defects are not corrected by the other partner's DNA.