GEN • GEN: Michael Cooley's Genetic Genealogy Blog
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18 May 2016
Why Test Autosomes?
Y chromosomal testing is particularly useful for several reasons.
Because it includes the male sex gene it passes down the
fatherline much the same way a surname is handed down. A male
Ashenhurst, for example, holds in his cells the Y-DNA fingerprint of
his earliest discoverable Ashenhurst "father." Because there are about 37
million chemical bases on the Y (the Big Y tests for 10
million), the Y is particularly fruitful. And because small mutations occur
every several generations—mutations that are passed down to seceding
generations—enough accumulate over dozens of generations and thousands
of years that distinct branches begin to emerge, branches that can be
identified through DNA testing.
Mitochondrial DNA inheritance works generally
the same way in that it passed through the motherline, and each of us
possesses the mtDNA fingerprint of our earliest "mother."
Unfortunately, mtDNA lacks the power of the Y in that there are only a
little more than 16,000 bases. Mutations are rarer, so branching occurs
much less frequently. And researching a mitochondrial maternal lineage is
more difficult because the surname changes every generation. For this
reason, the Y is easier to organize into surname projects. Still,
mtDNA has the power of the single lineage, that slender thread that
follows the mother's mother's mother going back to the beginning of
Here's a fan chart I found on the web years ago. It should illustrate the
Follow the bottom rib along the paternal Griffin line to the left and the
maternal lineage and its ever-changing surnames to the right. The DNA
fingerprints will match for each person along its respective arm. From this
graphic, we can readily see that genetic information for the other ancestors
cannot be gleaned from the Y chromosome or mtDNA. But there's still
Every one of our cells contains 23 pairs of chromosomes, the most
famous of them being the so-called sex chromosomes, the X and the Y.
Many of us remember from high school biology that women have two Xes, one
from each parent, and men have an X and a Y. The X comes from one of the
mother's two Xes and the Y, obviously, from the father. The remaining 22
chromosomes are called autosomes.
A set of autosomes comes from both parents, each comprised of a single
set of bases, the genetic alphabet soup of A, C, T, and G.
For example, at any specific position of, say, chromosome 7, a T might have
been inherited from the father and a G from the mother. Sort out these
pairs and stretches of genetic sequences can be determined that go up any
branch of the tree, not just the maternal and paternal branches.
Autosomal tests look at about 700,000 base pairs. These particular bases
are selected primarily because they tend to have a degree of variability.
(Without variability, we'd be unable, naturally, to see genetic
differences.) However, it's impossible to determine which bases came from
which parent only by looking at the results. They're not marked "this one
came from your dad and this one came from your mom." Progress cannot be made
without comparing the results among individuals. The larger the database,
DNA testing companies do most of the work for you by sorting, evaluating,
and displaying your matches. Your results are compared across their entire
database. They determine the size and number of matching segments, arrange
them from the best matches to the poorest, and determine a range of possible
FTDNA's autosomal product is called Family Finder ($99). Here are my top
matches (minus the names of the matching individuals):
And at 23andme:
"cM" stands for centiMorgans, which is simply a unit of
measurement within a strand of DNA.
As mentioned, we inherit half of our autosomes from each parent. A
quarter, therefore, comes from each grandparent and an eighth from each
great-grandparent. But which chunks of DNA came from whom? We can know
that only by comparing to other testers. My sister and I share a quarter of
our DNA but there's nothing in the results to tell us from which parent the
various bits were inherited. Clues are present in the DNA results of our
maternal half-sister because we share DNA with her only through our mother.
Identifying maternal markers helps to determine which side of the family a
distant cousin comes from. The same can be accomplished with any close
degree of relationship.
Because DNA from any one ancestor halves with each generation, relatively
few generations pass before what remains is so minimal that the chances of
finding a match becomes remote, particularly considering that any two
distant cousins might have inherited entirely different strings of bases
from the mutual ancestor. Therefore, autosomal results are typically
reliable to only about five or six generations. Nevertheless,
genealogical brickwalls can fall should enough data be collected.
I'll provide specific case studies in an upcoming post.