For genealogical research you need something more than just a tree builder, you need a system to record facts based on documents (like birth certificates) and a mechanism to link those records (I think person A mentioned in birth certificate C the same person as person B mentioned in marriage certificate D). The family tree is the accumulation of these the documents and the assumptions you made. It is possible that when you encounter a new document, you have to drop some of your assumptions, because, for example, it does occasionally happen that a person with a same (or similar) name is born on the same date in the same city.