You (1st gen) - 2 parents
1st cousins (2nd gen) - 4 grandparents
2nd cousins (3rd gen) - 8 great grandparents
3rd cousins (4th gen) - 16 great great grandparents
4th cousins (5th gen) - 32 (3xgreat) grandparents
5th cousins (6th gen) - 64 (4xgreat) grandparents
6th cousins (7th gen) - 128 (5xgreat) grandparents
7th cousins (8th gen) - 256 (6xgreat) grandparents
8th cousins (9th gen) - 512 (7xgreat) grandparents
9th cousins (10th gen) - 1,024 (8xgreat) grandparents
10th cousins (11th gen) - 2,048 (9xgreat) grandparents
11th cousins (12th gen) - 4,096 (10xgreat) grandparents
12th cousins (13th gen) - 8,192 (11xgreat) grandparents
13th cousins (14th gen) - 16,384 (12xgreat) grandparents
14th cousins (15th gen) - 32,768 (13xgreat) grandparents
15th cousins (16th gen) - 65,536 (14xgreat) grandparents
16th cousins (17th gen) - 131,072 (15xgreat) grandparents
17th cousins (18th gen) - 262,144 (16xgreat) grandparents
18th cousins (19th gen) - 524,288 (17xgreat) grandparents
Number of Parental Relationships: NPR = 2^x (2 to the power of X, X is how many generations back from yourself)
To go back 19 generations (for 18th cousins), NPR = 2^19 = 524, 288 parental relations - over half a million moms and dads to produce you!
You and an 18th cousin would then share 1/524,288th or 0.0000019% of your ancestry.
This number is not an absolute though, as something called "pedigree collapse" reduces the actual number of individuals involved, as people may
inadvertently marry 4th, 5th, or 6th cousins (or 1st cousins as was common among royals like the Hapsburgs). This causes the same person to appear in
a family tree more than once. (see Pedigree Collapse
Today there must be millions of descendants of John Lackland, or for that matter any royalty of that era.
The claim that US presidents are all part of the same family that is ruling the world looks mighty weak, as such a large number of persons (millions)
would have long ago squandered their resources and family holdings. Only a direct line of descendants would have retained the family titles, property,
etc. 18th cousins would be so far out of the picture that whatever royal line spawned them would hardly bother to recognize them as a relation. I saw
on Yahoo answers;
Dr Millard has calculated that Edward III probably has almost 100 million descendants. So it would be a very rare person of English descent (in
England, Europe, or North America) that is not a descendant of Edward III.
However, the overwhelming majority of people cannot trace their lineage. It is fun to be able to actually look at the tree.
Barack Obama can trace his lineage back to Edward I (the grandfather of Edward III).
With regard to the previous statement about "royal ancestry". If you are partly of European descent, and you define European royalty as beginning
with Charlamagne in 8th century, it is certainly mathematically probably that every single person (no exceptions) is a descendant from one European
monarch or the other. Once again, that is a mathematical calculation. There is no way to prove or disprove the statement. In any even the majority of
people cannot trace their lineage.
Queen Elizabeth is a descendant of Charlamagne (no surprise there). What is shocking to some people is that we know of more than a million lines of
descent from Charlamagne to QEII.
In this respect most of the people in this thread are related when going back 18, 19, 20 generations. Then again, yet another theory that claims our
family trees don't keep expanding forever mathematically as that would mean trillions of people would have to have lived to sire you, even as recent
as 1200AD, this one is called "Diamond Theory of Ancestors", here
how your family tree can narrow beyond a certain number of generations.
My take on all this is, 18th cousins might as well not be related at all, after 3rd, 4th, or 5th cousins it doesn't really matter. Unless Obama or
Bush or however benefited from these relationships by inheriting titles or wealth or castles then their being 18th cousins of some royal who lived
400-500 years ago is pointless drivel.
Not to take anything away from the little girl who did all this work though, she's got a bright future let's hope! (As a person of royal descent
myself (um, 21st cousin to Fergi?) this then is my official decree!