Excuse my ignorance as I haven't paid much attention to these DNA tests although always been curious.

I understand the concept of the ethnicity by geographic region and matching distant family members because of their DNA profiles, when they have all registered with Ancestry or whatever provider they use.

What I don't understand is how people find out because of these tests that their father isn't their father or they have half brothers etc when those people haven't done the DNA testing and are therefore not on the database?

Apologies if this is something obvious but I had no idea this was something these tests unearthed until I read this thread.