If they are not known, a father and mother can be setup as Birth Father and Birth Mother and they be marked as PREFERRED PARENTS.
Is this a person from the 1800's or a contemporary person living today?
Some people might handle the situation differently depending on that. Some people would consider a person they personally know as "part of the family" and prepare their reports accordingly. Whereas, the same situation looked at through the "cold" lens of 200 years of history would tend to look more strictly at the blood line.