Cheek/Chick DNA Project

Genetic Distance

"Genetic distance" is the number of mutation "steps" or mismatches between any two individuals.  "0" is a perfect match, "1" is a one-step mutation, etc.  The more mutations, the longer the probable time period since the most recent common ancestor.  For purposes of genealogy, we are mainly interested in knowing whether we are descended from a common ancestor in a "genealogical timeframe" (usually 300 to 500 years).  Knowing that we had a common ancestor 2,000 or 5,000 years ago may be interesting, but it's not helpful for genealogy because no records exist that can pinpoint any particular ancestor.

Generally speaking, a genetic distance of 0, 1, or 2 on a 25-marker test, in people with the same surname, is considered a "match," with a high probability of a common male-line ancestor in the last 300 years.  A genetic distance of 5 or greater indicates that the individuals did not have a common male-line ancestor within a genealogical time frame.  A genetic distance of 3 or 4 on a 25-marker test is inconclusive: the individuals could be descended from a common ancestor in less than 300 years, but there needs to be additional evidence to establish the connection.

Important points to keep in mind:

• Some markers are "linked."  For example, markers #389-1 and #389-2 tend to mutate together, so a difference on both markers is considered a genetic distance of "1" rather than "2".  The Family Tree DNA calculations take this into consideration.
• Markers mutate at different rates.  Family Tree DNA calculates the time to the most recent common ancestor using complicated mathematical formulas developed by scientists at the University of Arizona.  The formulas take into consideration not only the number of differences between any two individuals, but which markers are different.  Every marker has a unique mutation rate.  Some markers are highly stable and change little over generations, but other markers mutate frequently.  A mutation on a "fast-mutating" marker like #449 does not mean as much, genealogically speaking, as mutation on a stable, "slow-mutating" marker like #19.
• Unusual situations are a problem.  The mathematical formulas cannot handle unusual situations, such as the duplications of markers found in a couple of our participants, or the extremely rare "20" on marker #464d in Cheek Family Group 1.  This makes the calculations less accurate.  Although we know that if two people both have a duplication or very rare marker, the probability of a recent common ancestor goes up (i.e., becomes even more likely), but there's not enough data to calculate the exact probability in those cases.
• Probabilities are not certainties!  These numbers are only probabilities.  For example, an exact match on 25 markers means there is a 94.2% probability of a common ancestor within 12 generations (roughly 300 years) or less.  On the other hand, there is a 5.8% chance—small, but not insignificant—that the common ancestor lived more than 12 generations ago.  This is why genetic testing always needs to be combined with traditional genealogical research to help pinpoint the common ancestor.
• Coincidences can happen.  Sometimes, people with completely different surnames and no evidence of a genealogical connection can have a perfect DNA match.  This can happen simply by coincidence in people from the same ethnic group.  Europeans, in particular, have less "genetic diversity" than people in many other ethnic groups.  This is probably because the ancestors of Europeans experienced several population "bottlenecks" like the Ice Ages which drastically reduced their population size.  Like people in an isolated Appalachian valley, they had to marry their cousins because there wasn't anyone else.  If this happens for many generations, everyone in the population will have similar DNA.  Coincidental DNA matches are especially common on 12-marker tests, which is why 25 or 37 markers have become the standard in genealogy.