The Genetic Distance is a measure of the variance between two Haplotypes (or more simply, the variance between a set of yDNA STR test result values from two testers). In simplest form, it is the sum of the absolute value difference between each STR value tested in each tester. But some allowance is given for special multi-value STRs. And a few STRs may be ignored or discounted further if from a highly varying group. Genetic Distance is still a subjective formula that varies depending on who provides the value.
As an example, see the two rows of markers from a y12 test shown here. Each row is a different tester. The lower count value in a marker, when two markers are different, is highlighted in yellow. The GD, calculated as the absolute difference, is shown below each marker result. This can be reported as a GD of 4 or possibly only 3 as DYS385 has actually skipped two values in a single generation. Using only a maximum difference of 1 on each marker is termed the infinite alleles method.
This calculated value, like the Generation Difference, is a simplification and should only be used as a rough guide. It is used by FamilyTreeDNA to determine if a tester appears in another testers match list. At each test comparison level, FTDNA sets a limit and only shows those testers with a GD within that limit.
Generally, depending on how many STRs are being compared, the GD should be fairly small if two testers are on the same patriline in the genealogical time frame. For a y12 test comparison, a GD of 0 or 1 is the usually the maximum you should ever expect if you are expected to share a recent, common ancestor. So the two testers, in the example above, are likely not related in any genealogical time frame (or last 500 years)).
Note that only this term (Genetic Distance) is abbreviated as GD; and not the Generation Difference term used in genealogy.
It is always important to stress that STR values can change back as easily as they change further away from a given value. That is, a 37 count value in a marker could go to a 36 count and then in a few more generations go back to a 37 count. This is termed convergence. So getting generational estimates to a common ancestor from the GD is more of a very rough estimate than a norm. And best applies to people related in the last millennium (1,000 years).
As with many terms, the biological meaning is more expressive than the adopted meaning used by the genetic genealogy community. Biological definitions measure the numerical GD of whole genomes, and not just STR measured results. There is also an application of GD used by some in the genetic genealogy field that applies to mitochondrial DNA but this is not covered here or by our project.
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Calculating the GD from two 12 STR Marker testers |
This calculated value, like the Generation Difference, is a simplification and should only be used as a rough guide. It is used by FamilyTreeDNA to determine if a tester appears in another testers match list. At each test comparison level, FTDNA sets a limit and only shows those testers with a GD within that limit.
Generally, depending on how many STRs are being compared, the GD should be fairly small if two testers are on the same patriline in the genealogical time frame. For a y12 test comparison, a GD of 0 or 1 is the usually the maximum you should ever expect if you are expected to share a recent, common ancestor. So the two testers, in the example above, are likely not related in any genealogical time frame (or last 500 years)).
Note that only this term (Genetic Distance) is abbreviated as GD; and not the Generation Difference term used in genealogy.
It is always important to stress that STR values can change back as easily as they change further away from a given value. That is, a 37 count value in a marker could go to a 36 count and then in a few more generations go back to a 37 count. This is termed convergence. So getting generational estimates to a common ancestor from the GD is more of a very rough estimate than a norm. And best applies to people related in the last millennium (1,000 years).
As with many terms, the biological meaning is more expressive than the adopted meaning used by the genetic genealogy community. Biological definitions measure the numerical GD of whole genomes, and not just STR measured results. There is also an application of GD used by some in the genetic genealogy field that applies to mitochondrial DNA but this is not covered here or by our project.
External References
- Explained at Wikipedia
- Explained at ISOGG (or more applicable, see the ISOGG description of the FTDNA match technique including in a surname project manager suggested techniques page
- Explained at FamilyTreeDNA
- Explained by Roberta Estes