It is bad to have incest relations.
Hank, a single man, goes to a family reunion and sees a lot of beautiful relatives.
Hank wants to find a potential wife while at the reunion.
Hank meets a fifth cousin by marriage, dates her, marries her, and has a child with her.
Hank meets a second cousin, eventually marries her, and has a child with her.
The child is normal and has no genetic abnormalities that anyone can see.
The child has a recessive genetic disorder because Hank and his wife share a defective gene.