Cheek/Chick DNA Project

Genetic Distance

"Genetic distance" refers to the number of mutation "steps" or mismatches between any two individuals.  "0" indicates 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 about 300 to 500 years.  Knowing that we had a common ancestor in the last 2,000 or 5,000 years may be interesting, but 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 within the last 300 years.  A genetic distance of 5 or more indicates that the individuals probably 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 other evidence to establish the connection.

Some important points to keep in mind when analyzing genetic distance:

• Markers #389-1 and #389-2 tend to mutate together, so a difference on both markers—as seen between some participants in Group 1—is considered a genetic distance of "1" rather than "2".
• Family Tree DNA estimates 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.
• The mathematical formulas are not equipped to handle unusual situations, such as the duplications of markers found in a couple of our participants, or the extremely rare "20" on marker #464d.  This adds a degree of uncertainty to the calculations. Although we know that if two people both have a duplication or very rare marker, the probability of a common ancestor goes up —but there's not enough data to calculate the exact probability.
• These numbers are only probabilities.  For example, an exact match on 25 markers means there is a 94.2% probability the common ancestor lived within 12 generations (roughly 300 years) or less.  Conversely, there is a 5.8% chance—small but still not insignificant—that the common ancestor lived more than 12 generations or 300 years ago.  This is why genetic testing always needs to be used in conjunction with traditional genealogical research in order to help pinpoint the common ancestor.
• The more markers tested, the more accurate the calculations. For example, Participant #24 differs from the most typical Group 1 profile on markers #389-1, #389-2, and #459a, and he also has a duplication on #464d.  Based on the 25 marker test, there is only a 31% chance that Participant #24 and the "most typical" Group 1 participants had a common ancestor within the last 12 generations (approximately 300 years).  However, when all 37 markers are compared, the probability of a common ancestor within 12 generations rises to a much more comfortable 80%.  Click here to see the 37-marker table.
• Sometimes, people with completely different surnames and no evidence of a genealogical connection may find that they have a perfect DNA match.  This can occur 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 the population size.  Coincidental DNA matches are especially common on 12-marker tests, which is why 25 or 37 markers have become the standard in genealogy.