I received the official word today from FTDNA on this. The information is in the information on a project administrator GAP main page. The two genetic distances (Y-111 and Big Y-500) should be added together for comparisons and not considered separately. The 111 marker genetic distance is a measure we have known for a long time. What is new is the Big Y-500 genetic distance column (which is headed "Big Y-500 STR Differences") refers to STRs in addition to -- and NOT including -- the Y-111 STRs. FTDNA wants to find at least 389 additional STRs that are readable. So they test 450 more STRs, reading them 70 times and considering them as readable if they give the same result 10 times. So the specific additional STRs that are readable for person A may not be the same STRs readable for person B, so that person X who is being compared to the A and B might differ from A on STRs that B did not even have readable. The 111 marker test is an apples-to-apples comparison of the same 111 STRs for both persons. So a difference in any of them is apples-to-apples. But a difference of the Big Y-500 using only the genetic distance number might be apples-to-oranges, if the differences are in different STRs where A differs from X on an STR that B does not even have among his readable additional STRs. You really need to go into the detailed results and see just which STRs are involved in the match of X and A and in the match of X and B. ------------------------------ Date: Tue, 4 Dec 2018 10:54:22 -0600 From: Robert Casey <robert@rcasey.net> Subject: [DNA] Re: GENEALOGY-DNA Digest, Vol 13, Issue 546 Date: Mon, 3 Dec 2018 20:01:15 +0000 (UTC) > From: Wesley Johnston <wwjohnston01@yahoo.com> > Subject: [DNA] FTDNA Y-STR vs Big Y-500 Genetic Distance differences > > There must be some explanation somewhere on the FTDNA web site to explain this, but I cannot find it if there is one. > The new format of Y-DNA matches includes a new column "Big Y-500 STR Differences" which is thus a genetic distance column. > I clearly do not understand how best to interpret and use the Big Y-500 information. Comparing the prior and new genetic distance columns, some things do not make sense, which almost certainly means that my understanding of the Big Y-500 is flawed. > 1 - Some Y-111 matches have a genetic distance that that is greater than the Big Y-500 difference. For example, one matching kit has a Y-111 genetic distance of 6, but a Big Y-500 of "1 of 442". And another is Y-111 at 5 and Big Y-500 at "2 of 437". I had thought that the Big Y-500 STRs included those in the Y-111 plus some more. If that were the case, then the Y-111 genetic distance would always be less than or equal to the number of differences in the Big Y-500. But clearly that is not the case. So what is the relationship of the STRs in the Y-111 test to those in the Big Y-500 STR results? > 2 - The other thing that sticks out very noticably is that the closeness of matches at Y-111 genetic distance is sometimes the exact opposite of the number of Bug Y-500 differences. For example, three kits match at a Y-111 genetic distance of 3 (call it Kit A), 5 (B) and (6). But in the Big Y-500 STR differences, they are 5 (A), 2 (B) and 1 (C). How should I interpret and use this significant difference in order? And which order is the "real" order"? >  The first number is genetic distance. But this is "Infinite Alleles" genetic distance where any change of a marker value counts as only one mutation (multi-step mutations count only as one mutation). Any small twist, Y500 has a lot of no-calls. This genetic distance is only for markers that are called by both testers that are being compared with each other (no calls of either tester remove the YSTR from the analysis). The second higher number is a form of testing resolution. It is the number of called mutations that are shared. So if two testers both have few no calls - the number will be higher and resolution will slightly higher. This would result in slightly higher accuracy due to a higher sample size. Also, if real low, some key mutations could be missing and manual inspection may reveal that key markers are missing in the comparison.