Genetic Distance

When two people learn that they might be related to each other because their Y-DNA have a close match, they wonder how close their relationship might be. Y-DNA results can not specify an exact relationship between the two people. The best it can do is give a general idea of their relationship. This is because the mutations that occur in the Y-DNA occur randomly. In addition, some values of Y-DNA mutate more often than other values.

There are several companies doing DNA testing for genealogical purposes. The two major companies are Ancestry and FamilyTreeDNA. These companies have different ways of specifying an approximate relationship between two people. The Ancestry site gives the number of generations to the most recent common ancestor (MRCA) for a 50% probability. The probability factor is involved since Y-DNA can't give exact relationships. FamilyTreeDNA gives the genetic distance between the two people, where genetic distance is the number of differences in the values of the markers of the two men. For example, if the genetic distance is 2, there are two ways that genetic distance could exist. The two men might have two DNA markers that differ by a value of 1 for each marker, or they might have one DNA marker that differs by a value of 2. Without additional DNA testing there is no way of knowing when and in which line(s) the mutations occurred.

This post gives information from FamilyTreeDNA that will help us estimate the closeness of a relationship based on the genetic distance of the two men. The more markers that were tested, the wider the range of genetic distance that can exist and still have a relationship. Here are links to the information from FamilyTreeDNA. The following information on relationships was taken from the FAQ at familytreedna.com (http://www.familytreedna.com/faq/answers/default.aspx?faqid=9#913) That page is worth reading, because it will clarify interpretations of DNA results.

	Y-DNA12	Y-DNA25	Y-DNA37	Y-DNA67	Interpretation
Very Tightly Related	N/A	N/A	0	0	Your exact match means your relatedness is extremely close. Few people achieve this close level of a match. All confidence levels are well within the time frame that surnames were adopted in Western Europe.
Tightly Related	N/A	N/A	1	1-2	Few people achieve this close level of a match. All confidence levels are well within the time frame that surnames were adopted in Western Europe.
Related	0	0-1	2-3	3-6	Your degree of matching is within the range of most well established surname lineages in Western Europe. If you have tested with the Y-DNA12 or Y-DNA25 test, you should consider upgrading to additional STR markers. Doing so will improve your time to common ancestor calculations.
Possibly Related	1	2	4	7	Without additional evidence, it is unlikely that you share a common ancestor in recent genealogical times (1 to 6 generations). You may have a connection in more distant genealogical times (7 to 15 generations). If you have traditional genealogy records that indicate a relationship, then by testing additional individuals you will ether prove or disprove the connection.
Probably Not Related	2	3	5	8-9	It is highly unlikely that you share a common ancestor in genealogical times (1 to 15 generations). Should you have traditional genealogy records that indicate a relationship, then by testing additional individuals you will ether prove or disprove the connection. A careful review of your genealogical records is also recommended.
Not Related	3	4	6	10	You are not related on your Y-chromosome lineage within recent or distant genealogical times (1 to 15 generations).

As you use this information, don't "nit pick" the numbers. If the genetic distance is on the border of a possible relationship or no relationship, give yourself the benefit of the doubt and assume a possible relationship.

Even though Ancestry doesn't give genetic distance values, persons using Y-DNA values from Ancestry can still obtain genetic distance values by counting the number of markers that are different for the two persons, as explained above.