How Far Back Can You Get?
It’s a common notion that a family tree chart which lists ancestors going back to 1600 must be “better” than one that
“only” goes back to 1850. On the contrary, though, an-depth family history covering just a few generations may well
be more interesting – and more reliable – than a list of names stretching back for hundreds of years.
How many ancestors do I have?
Let’s see… two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents…
Generations Back
1
2
3
4
5
6
7
8
9
10
11
12
Ancestors in That Generation
2
4
8
16
32
64
128
256
512
1,024
2,048
4,096
Relationship to You
Parents
Grandparents
Great-Grandparents
Great-Great-Grandparents (Two Greats)
(Three Greats)
(Four Greats)
(Five Greats)
(Six Greats)
(Seven Greats)
(Eight Greats)
(Nine Greats)
(Ten Greats)
Is it possible to trace a complete family tree back twelve generations?
For a person born around 1960 (someone who turned fifty around 2010), going back twelve generations would take
you back, very roughly, to people born around 1600. You have about four thousand ancestors in the twelfth
generation. (In reality, you have fewer, because there is always some intermarriage between distant cousins; but four
thousand is probably close.) You have another four thousand ancestors who lived in the intervening generations (from
the eleventh generation down to your parents). So, going back twelve generations, you have a total of around eight
thousand direct-line ancestors. It isn’t possible to trace them all. For one thing, not enough records have survived.
For another thing, if you spent one day researching each of your 8,000 ancestors, that would take you 22 years.
Can a single line be traced back twelve generations?
Well, sometimes. But there is always a possibility for error. Sometimes two unrelated people who had the same
name lived in the same small town. Sometimes an adopted child is raised with no knowledge of his or her biological
parents. In these cases, all of the available evidence might point to a single, very justified conclusion about ancestry –
a conclusion which is quite wrong. The chance for error is magnified in genealogies stretching for many generations.
But the blood of my distant ancestors flows in my veins!
Does it, always? You have four thousand ancestors who lived twelve generations ago. Do you really get exactly one
four-thousandth of you DNA from each? No, because during the copying process, “clumps” of DNA tend to stick
together. Of your four thousand ancestors in generation #12, it is statistically likely that only about 125 of them have
passed DNA on to you. See http://www.genetic-inference.co.uk/blog/2009/11/how-many-ancestors-share-our-dna/.
But can’t genetic testing establish a link to the distant past?
Only in certain cases, and with limitations. Suppose your last name is Dweebington, and your Y-chromosome (if you
are male) matches other living people named Dweebington. And suppose your genetic cousins trace their ancestry to
Lord Dweebington, who was born in 1612. Fine. But the DNA test only tells you that you have a common male-line
ancestor; the test doesn’t say who it was. Was it Lord Dweebington, or was it Lady Dweebinton’s footman?
The “pedigree collapse” problem.
A friend of mine claims to have traced her ancestry back to someone who lived twelve hundred years ago. That’s
about 40 generations back. How many ancestors do you have who lived 40 generations ago? The simple equation
40
says that the number should be 2 – a number which is far larger than the number of people who have ever lived on
earth. This is called “pedigree collapse.” In the distant past, people on different continents didn’t usually mix, so you
are not necessarily descended from everyone who was alive twelve hundred years ago. But you are descended from
nearly everyone who lived in the part of the world that most of your ancestors are from. So take heart: we are all
descended from royalty!
FamilySearch Center at the John Parker Library
c. hanlin 10.11