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Pedigree
Collapse
Ancestry is exponential.
Starting with you, your immediate predecessors are your two parents,
then each of them has two parents, etc. Continuing thus, then five generations
back, there are eight pairs of second great-grandparents or sixteen
lines. Looking back ten generations each of us should have 256 pairs
of seventh great-grandparents. Twenty generations back each of us is
theoretically related to 524,288 sets of seventeenth great-grandparents
which is how many lines? It works out to one million, forty-eight thousand,
five hundred and seventy-six! The problem with this perfect family tree
is that twenty generations back (about 1400 AD) the population does
not support these numbers.
Obviously there
were never one septillion people on this earth, and there never will
be--the number is just too huge to contemplate. So how do we explain
the perfect exponential-increase of family trees? The answer is in the
marriages of cousins. We all have marriages between cousins in our genealogies,
and usually we do not have to go too far back to find them. It is the
marriage of cousins that creates what is known as a pedigree collapse,
meaning that the number of unique individuals in any family tree is
reduced because one may be related to a particular ancestor in one or
more different lines, or put another way, a particular ancestor may
appear in more than one place in a family tree. For example, if one's
parents are first cousins, then there are only two grandparents instead
of the usual four, which in this case reduces the number of ancestors
by 50%. When it comes down to it, we are all descended from only two
people--our parents!
Pedigree collapse
occurs in every family tree, and in some there can be interesting results.
There is an example of one family tree researcher finding himself as
a direct descendant of six children from the same 18th century family.
It is not unusual to find oneself ascending from two, if not more, lines
of one surname or branch. You could quite conceivably find cases where
an individual is related to you on both sides of your family and, yes,
several more perhaps from several different grandparents and great-grandparents,
to stretch the imagination. It happens so often that when you see your
surnames of interest on the Internet, investigate them because it's
usually a rare occasion that you don't find at least one relative, particularly
if they are in the same area of your research (thus the intrinsic value
of joining SHARE!)
Family trees work
both backwards into the past and forwards into the future. If going
back sixteen generations means that one is theoretically descended from
32,768 people, think how many descendants each of those ancestors has.
One could conceivably uncover more than 1,500 people descended either
by blood or marriage from one 16th-century Englishman. Assuming he is
representative of most heads of Elizabethan English families, multiply
those 1,500 descendants by the 32,768 people representing 13 generations,
and one is faced with the possibility of being related to more than
49 million modern-day people. One could be related to a friend, a neighbour,
a co-worker, etc. It has been suggested that half of present-day North
Americans are descended from a shared ancestor who lived in the 1900s.
The term "family of mankind" makes a lot of sense when one looks at
it this way.
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