The number of members in each generation in a family tree is double the number of members in the previous generation.
So, starting from current generation with 1 member,
Second generation =1×2=2 members
Third generation =2×2=4 members
Fourth generation =4×2=8 members
Fifth generation =8×2=16 members
Sixth generation =16×2=32 members
Seventh generation =32×2=64 members
Eighth generation =64×2=128 members
Therefore, the eighth generation of a family tree has 128 members.