It has become apparent that some testers feel that testing for low numbers of markers is adequate once a match has been discovered. This can sometimes be a false promise and it is not always easy to explain.
It's generally true that you can 'see' a relationship at 25 markers if you can distinguish it from all the other dross that testing at this level throws up. 25 marker testing tends to produce lots of false positives which can only be resolved with a greater number of tests.The greater the number of tests, the greater the statistical significance. FTDNA considers a genetic distance (GD) of 2 as the significant limit at 25markers, 4 at 37, 7 at 67 and 10 at 111. It then depends on which actual markers are different. Say you have a GD of 5 @ 67 markers which is highly significant; if these differences occurred in the first 37 markers, you probably wouldn't see this relationship at 25 or 37 markers (because of FTDNA cut-off points). Consequently, we encourage new and existing members to test to 67, and ideally 111, markers. This is the bedrock of your profile and is used in all projects and studies.
Another problem facing testers is the array of tests available which often causes confusion. Projects like this one, which follow patrimonial inheritance, can only use Y-chromosome DNA data. For example, unless you are following matrimonial inheritance for other reasons, testing for mitochondrial DNA is unnecessary. Having Family Finder and/or mt-DNA tests performed will not help you track your paternal ancestry and will be ignored in this Project
Next in importance to STR testing (above) is SNP testing to establish your haplogroup. From your STR results, FTDNA are able to predict your most likely broad haplogroup. This is shown in red and is normally R-M269 for most McMahons and corresponds to the evolutionary stage of early Neolithic Europeans. Big Y, Pack or individual SNP testing is needed to establish further evolutionary branching typically to R-DF13 or R-DF21 for most McMahons (actual test results are shown in green). Many have tested for more recently discovered SNPs which are now being used to define the Project haplogroups.
Another cause for concern among some testers is where no matches are found even when testing to 67 markers. This should not be regarded as a negative result.It is common and simply means that your lineage evolved early (and is outside the FTDNA statistical limits), no other distant relatives have tested or you are the only survivor of your lineage to have made it to the 20thcentury. Sometimes joining other name projects can result in a match.
Over time a fair number of non-McMahon matches have come to light. Some of these are known adopted names while others are suspected of being adopted names or non-parental progeny. Prior to record keeping (mid 19thc.), fostering sons between chieftains was a common practice which no doubt could have accounted for some switching of names and DNA. Before proper surnames were introduced (in and around the second millennium), it is also likely that some tribal/family groups had become genetically mixed before they were known by their surname.