There is a very slight risk of first generation incestually born children having medical problems. The bigger problems occur when it continues for a 2nd or 3rd generation.<quoted text>
Since I'm not a biologist or a medical professional, I have to rely on the Internet. Here's a bit I found... "Inbreeding leads to a higher probability of congenital birth defects because it increases that proportion of zygotes that are homozygous, in particular for deleterious recessive alleles that produce such disorders."
Another article states, "Doctors are worried about all close incestuous parings that result in a child, since such children have a much higher risk of disabilities."
Regarding incest and religion...
--According to most Biblical scholars, it is widely believe that Cain married his sister (since there were no other people on the planet).
--Abraham married his half sister Sara. They shared the same father, but not the same mother.
--Nachor (Abrahamís brother) married Melcha (his niece).
--Two of Lot's daughters got Lot drunk and had sex with him to preserve "the seed" of their father.
--Amram took to wife Jochabed his aunt by the fatherís side: and she bore him Aaron and Moses (so Aaron and Moses were the result of a marriage between their father and their father's aunt).
--Amnon, the son of David, fell in love with his sister, Thomar and had sex with her against her will.
Any child can have 2 recessive genes which lead to birth defects, not just children of incest. So the chances aren't any higher for the first generation incest born children than any other child.