At the end of the day, what difference does it make? You can't change the past. Also I'm pretty sure knowledge like "he's not my real father" will stay in the back of your mind and pop up from time to time, making things awkward for a long time.
I think maybe it boils down to some people always wanting to know (me) and others who don't (whatever it is) - and you've probably self-selected when you clicked all the boxes as you spat into a tube.
Exactly the situation I'm in, with both a parent and a grandparent, and it's always boiled down to these people have had a significant impact on my life, and these people have not. I don't understand the obsession with genetics - the thought "he's not my real father" has never once crossed my mind.