The author of "Santorum related sodomy to other laws on sex" (April 29 letter) wrote in support of the recent remarks of Rick Santorum (R-Pa.) saying that if sodomy laws are overturned by the Supreme Court, then other forms of sexuality between consenting adults would be protected, including incest.

This is not a logical extension of the law. Any moral system has as its fundamental principle: Do no harm. Homosexuality and incest are not equivalent with respect to a moral system that tries to prevent harm. Homosexual sex between two consenting adults does not cause harm to anyone, but incest clearly can cause harm biologically.

It is widely accepted that most human genomes harbor five to 10 recessive deleterious mutations. These mutations usually pose no threat when offspring are produced from two unrelated individuals because they are not likely to share the same sets of deleterious traits. On the other hand, closely related individuals, such as father and daughter, will have a higher probability of producing offspring that share deleterious recessive traits leading to genetic diseases that cause biological harm. Many human cultures have inbreeding taboos for this reason.

Thus, if the Supreme Court rules against sodomy laws allowing consensual homosexual relationships, then it does not follow that incest should be permissible because of the genetic consequences if offspring are produced.