It is a sobering thought that if we go back just 10 generations, each of us will have 1,204 ancestors. That's going back about 250 years, or to the year 1759.
If we could trace our pedigrees back 20 generations we would all have 1,048,575 ancestors:http://www.familyforest.com/resources.htmlIn practice however we do not all have 1,048,575 unique ancestors and many of our lines will be inter-related through close cousin marriages.
Ultimately we are all related to each other probably many times over. This concept has been called "pedigree collapse" or the "ancestor paradox"
:http://www.bpears.org.uk/Misc/AncestorParadox/http://www.generations.on.ca/genealogy/pedigree.htmhttp://freepages.genealogy.rootsweb.ancestry.com/~fettesi/unique.htmI don't know if figures are available for other countries but it is possible to come up with rough estimates for the population of England from the eleventh century onwards.
At the time of the Domesday Book in 1086 the population of England was estimated to be around one and a half to two million people:http://www.domesdaybook.co.uk/life.htmlIn 1348 the Black Death arrived in England and is believed to have killed over 50% of the population:http://en.wikipedia.org/wiki/Black_Death_in_EnglandThis page has figures for the population of Great Britain and Ireland from 1570 to 1931:http://homepage.ntlworld.com/hitch/gendocs/pop.html
The mathematics is beyond me but presumably it would be possible to do some sort of estimate as to how many ancestors we can all be expected to have in common. What effect do these complicated inter-relationships have on the inheritance of autosomal DNA?
Reference: from Debbie Kennett