F1 =2
F2=4
F3=8
F4<16 because among your 8 grand-grandparents, >=2 may from the same family, as long as they did not marry each other, and their children (your grandparents) did neither, but their grand children can marriage each other (your parents). it is like your grand children and your brother/sister's grand children may get married.
F5<32
The same reason as F4
.....
So the actual family tree will have much fewer origins than 2^n as n become greater.
