To prepare for my class, I wanted an estimate of how many ancestors a person has. Ignoring the possibility of duplicates, the math isn’t hard. For each additional generation, you double the number and keep a running total. For instance, you have 2 parents, 4 grandparents, and 8 great-grandparents, giving you 14 total ancestors in the three generations before you were born.

But how often does a new generation come about? That was the big question. I tried 3, 4, and 5 generations in 100 years, then stopped after 400 years because the number became so large.

If a new generation occurs every 33.3 years, you have 4096 ancestors in 400 years.
If every 25 years, the number is 65,536.
If every 20 years, the number is 1,048,576.
Imagine how many ancestors you have if you go clear back to Adam and Eve!