Michael Cooley's Genetic Genealogy Blog GEN • GEN
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 humanity.

Here's a fan chart I found on the web years ago. It should illustrate the point:

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 hope!

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 Testing

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, the better.

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 relationships.

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.