Mix and Match:
DNA Stories from an Ancestor
‘Satiable Curiosity is a column dedicated to the proposition that genetic genealogists are an untapped resource for resolving questions about DNA behavior--how
DNA changes over the course of a few or many generations and how DNA patterns
are distributed around the world. Some
questions are so broad that it could take
decades to arrive at a
conclusion, yet others are narrow enough
to answer in a shorter time frame, perhaps even
within a semester or two for a student research project. The
results may nonetheless be of considerable genealogical utility and scientific interest, worthy of publication in a technical journal.
genealogists are beginning to explore the whole genome–we are no longer limited
to the Y chromosome or mtDNA, which have served as the foundation of the
field. Companies such as 23andMe and deCODEme scan
hundreds of thousands of markers, which have come down to us from many
Think of a
family reunion where people are reminiscing about anecdotes their
great-grandmother told them. One might
say, "Do you remember the story about falling in the creek two times in
the same day?" Some might nod their heads, but others might say, "No,
I never heard about that, but how about the summer she made seventy-three pints
of applesauce from the tree in the back yard?" It's sometimes a matter of chance which
people were hanging around at the time the stories were told.
DNA operates in much the same way, as some of
the stories are passed on to the next generation. Descendants will inherit different fragments
of great-grandmother's DNA–sometimes
a number of descendants might receive the same section, and sometimes only one
descendant will carry a section, purely as a matter of chance.
A case study
illustrates how a particular story might be tracked down to its genetic source
by comparing DNA of descendants. A man and his second cousin both inherited a
distinctive trait from their great-grandmother.
The grandparents and parents from both lines were deceased, but the man
had two siblings who did not possess the trait.
Each of the siblings can be compared one at a time to the second
cousin. Figure 1 shows three
diagrams from the Family Inheritance feature at 23andMe (deCODEme
has a similar display), where sections of DNA found in both parties are color-coded
Figure 1. Family Inheritance diagrams from a comparison
of three siblings with a known second cousin.
Sibling 1 shares a particular trait with the second cousin, while
siblings 2 and 3 do not.
shares five segments of autosomal DNA (on chromosomes 1, 2, 7, and 18) with his
second cousin. This is actually somewhat
less than typical for second cousins, but it's handy for our purpose. Some of these could have come from their
great-grandfather or be unconnected to the trait in question. How can we narrow down the choices? If
Sibling 2 or Sibling 3 also share the segment, then we
know the trait was not located on that segment (since they don’t have the
trait). These sections are outlined in
red boxes. That leaves two sections,
outlined in red circles, which are found only in the sibling with the
trait. The genes in these segments are
good candidates for the mutation that caused the trait. Testing additional cousins with or without
the trait could eliminate one of these.
people in this case study already knew how they were related, the Family
Inheritance diagrams show that second cousins can be easily detected by the
amount of DNA they share. Their stories are not always passed on to the
next generation, but even so, third cousins usually
share a few segments. Fourth cousins may
also demonstrate segments in common, although not as frequently. Even more distant cousins will share some DNA a small percentage of the time. This opens up a whole new way of detecting
Ann Turner has
a consulting agreement with the company 23andMe, Inc. The opinions expressed in this article are
entirely her own.
DeCodeMe Web Site