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# A person has two parents, four grandparents, eight great-grandparents and so on backwards through the generations. If we go back 40 generations, about 1200 years, this gives each person on Earth over a million million ancestors, many times more than the population of the entire Earth! Where does the logic break down? Are we all very inbred?

Question #39058. Asked by gmackematix. (Sep 23 03 9:22 PM)

sequoianoir

Nice question - something that initially seems a bit of a paradox.
Took me a few minutes to get my head round it.
The calculations (2 power 20 = 1 million+ / 2 power 40 = million miliion+) are assuming that neither of my parents share a common ancestor anywhere in 40 generations.
A common ancestor say 7 generations back, about 200 years, who is greatgreatgreatgreatgreatgrandfather to BOTH your mum and dad, removes a huge quantity from the (everyone unique) 40 generation total.
(and obviously the nearer to the present this happens and the more frequently it happens thoughout the generations, great swathes of unique ancestors "vanish" as they become one and the same person)
This obviously means that there is inbreeding (but not very) because as this 7 generations back common ancestor passed his genes down to your mother, (and father) at each step they halved. This means that your mum has LESS than 1% of this ancestor's genes, as does your father.
If you received half again from each parent, then you would "maintain" around the same level. If by some fluke each parent passed on all of their common genes, then you could "double up" and then have 1.56% of this distant ancestor, twice that of your parents.

 Sep 24 03, 6:45 AM
taesma

The official term for this is Pedigree Collapse.
Here's an article:

http://www.generations.on.ca/genealogy/pedigree.htm

 Sep 24 03, 7:06 AM
gmackematix

Excellent research DB and taesma. I'd never fully got my head round this and had certainly never heard of "pedigree collapse". In early human history I suppose there was no option but to inbreed. As argued in "Inherit the Wind" the Bible tends to draw a veil over where the wives of Adam and Eve's sons came from!

 Sep 24 03, 12:19 PM

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