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FTDNA genetic distances wrong?

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  • FTDNA genetic distances wrong?

    I have manually checked a number of genetic distances using FTDNAs descriptions of how gd's are supposed to be calculated, and found that the gd's that FTDNA gives are sometimes not consistent with the manual calculations and seem incorrect. For example, here are the first 25 STRs for 2 kits.

    13 24 14 11 14-11 12 12 13 13 13 29 18 10-9 11 11 26 15 19 29 15-16-16-17

    13 24 14 11 14-11 12 12 13 14 13 30 18 10-9 11 11 26 15 19 29 15-16-17-17

    The genetic difference between the two kits seems to be 3. However, the FTDNA genetic distance calculator says that the genetic distance is 2.

    Here's another example, using 2 different kits and a 37-STR comparison:

    13 24 14 11 14-11 12 12 13 13 13 29 18 10-9 11 11 26 15 19 29 15-16-17-17 9 11 19-23 16 16 18 17 34-38 13 12
    13 24 14 11 14-11 12 12 13 14 13 30 18 10-9 11 11 26 15 19 29 15-16-17-17 9 11 19-23 16 16 18 17 34-38 13 12

    The genetic distance seems to be 2, but the FTDNA genetic distance calculator says it is 1. Does the FTDNA calculator have a bug? Or am I missing something?

  • #2
    I found out the reason for the difference between the manual calculations and FTDNA's calculations. It results from how FTDNA treats the DYS389i and DYS389ii markers. In the top example I gave, the 13/14 discrepancy is in DYS389i and the 29/30 discrepancy is in DYS389ii. Those markers are listed separately in the YDNA results charts (and are even separated by an unrelated marker). However, as explained in http://forums.familytreedna.com/showthread.php?t=29826 and http://www.johnbrobb.com/Content/DNA/TMRCA&GD.pdf, DYS389ii = DYS389i + DYS389B (the latter of which is not reported in FTDNA charts). Therefore to avoid double counting a difference in DYS389i, FTDNA subtracts DYS398i from DYS389ii in calculating genetic distances for DYS389ii (which results, essentially, in the calculation being the differences in DYS389B). Applying this calculation method in both of the examples above reduces gd by 1 and they match FTDNA's calculations.

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