A SNP (Single Nucleotide Polymorphism) test confirms a haplogroup designation by determining if a SNP has mutated from its derived or ancestral state. The following cases illustrate various uses for SNP testing.

Refining Ancestral Origins 
     The most important aspect of all, SNP tests serve to refine a persons' genetic signature by putting him/her within a selective group with others who are similarly tested and found to be positive for the same marker(s). In my case for example, at 12 markers, I was "estimated" by FTDNA as E3b. After 25, then 37 markers tested, I was still estimated as E3b and possibly E3b1, based entirely upon how close my genotype appeared to those whose SNPs had been established formally. It took the SNP test itself to prove the point. I then fell into my own little cluster, forever identified with some but not all the other E3bers.  
     So why is that so important? For one thing, it narrows the array of persons against which I need to compare myself. I can ignore anyone who is not SNP positive the same way for I am. As to those whose SNPs are different from mine, there is virtually no chance we are related within multiple thousands of years. I can also exclude those who appear related because of convergence - the random change in one or more STR markers - because they too are not related. In other words, even if SNPs can't tell me precisely who I am, they surely inform me who I am not.  
     For another thing, I can better understand the likely origin and movement of people (tribes?) in my ancient "Y" history. Here is what the SNPs I've tested positive for tell me about their age:

What this means is that I am positive for M168, M96, P2 and 391p, M35, M78, and V13.   
     Each time I tested positive for a SNP, the calculated timeline to its origin brought me closer to recorded history. The last SNP in this group was V13 and the best estimate is that it arose about 11.5 kbp. The next mutation will probably bring me to the late bronze, early iron age. Moreover, the collectively shared view among those researching my haplogroup is that the geographical origin of V13 is the present day Balkans. This means that my ancestors almost assuredly were there around 9,500 B.C. from which they moved in a westerly direction, ultimately arriving in Portugal. I have a paper trail tracking my Y chromosome to the 1670's in northern Portugal and two surnames (Silva and Affonso) that were within this area at least since the 11th century.
     Not everyone would find all this interesting or relevant to their genealogical viewpoint, but I do. Not everyone would agree these time and location estimates are accurate, but my response would be they are as good as the scientific community can offer at this point and that's not bad given there are precious few other options available. So that, in brief and sum, is why I think SNP tests are worth the money.
- Contributed by A. Silva

Confirming NW Irish Modal
The McCabe Surname DNA study, contains haplogroup results most of which are identified as R1b1.  A participant recently had the follow-up tests (specific SNP) provided by FTDNA and found that he was R1b1c7, which (1) means that those who  match him are also in R1b1c7 and (2)  indicates that their roots are specifically in the area of northwestern Ireland, Ulster and in lowland Scotland. 
     This is the haplogroup of the descendants of Niall of the Nine Hostages, Niall Noigiallach, the hypothesized High King of Ireland in the Fourth-Fifth Century and ancestor of the Uí Néill dynasties.  This result makes the story "come alive" for those three McCabes who have this specific sub-grouping of the R1b haplogroup. This result, as indicated above, provides tremendous help in pin-pointing the ancestry of those who match the R1b1c7 haplogroup. 
- Contributed by James M. Freed - Administrator for the McCabe DNA Project

Extremely Rare DNA
A couple of years ago there was a cluster of several dozen people who tested positive for M201, so they were within Haplogroup G, but they were found to be negative for every SNP within G that was then being offered commercially. Finally, a few members of this group were tested in a small research study for what was thought to be an extremely rare SNP, M377, defining Haplogroup G5, which had only been observed previously in two Pakistani men. Now the European branch of this haplogroup have something that clearly unifies them and adds to their sense of identity. Essentially all of this group are Ashkenazi Jews from Eastern Europe, though some did not previously know their origin.
- Contributed by Whit Athey

Convergence
In the Talley project we had 3-4 people whose SNP couldn't be determined without doing the test. This helped us determine if those with no haplogroup predictions were related, even remotely or not. It also showed us there would be a new haplogroup for the surname. SNP confirmation is the top level for division. Although rare, it indicates when there is a matching haplotype that isn't related, but happened to mutate into the same set of marker results. This is known as convergence.
- Contributed by Emily Aulicino - Administrator for the Talley DNA Project

Ancient Connections
As a Newbie, I am still learning the relevance of SNP testing to my overall genealogy and genetic ancestry.
     I have just paid for FTDNA to upgrade the SNP tests for my dad, a gt uncle (within one of my maternal one name studies - with a rare name) and a very distant cousin (now deceased) on the same rare name. The results in so far are fascinating...albeit I am struggling to really put the results in perspective.  The tests in our money were very expensive but I felt that by having them refined adds a 'completeness' to the family DNA file. It allows me to continue reading the deep DNA ancestry books coming out with a sense of involvement and satisfies somewhat a sense of curiosity when I see so many ancestry / history related TV documentaries regarding even the last 1-2,000 years that we get especially from the UK. I understand SNP is relevant to millennia ago, but somehow it helps me perhaps think where my origins might have tracked until recent history (such as Celt / Roman / Scandinavian migration) .....maybe such a belief is unfounded.
     In summary, a rare SNP as it now appears my mother and her paternal ancestry has tied into a rare surname, which is cause for a fascinating but difficult genealogical task. Compared to my own which is simply the most common UK / European yet is no less fascinating if it can be related to an understanding of life and survival in those (possibly tribal prehistory) times.
If the SNP goes back so far, how has the migration of my deep ancestry changed my genetic / physical attributes to become what I am today...fair skin, preference for cooler climes, hair / eyesight, these are some of my curiosities and a SNP might be a clue...maybe.
- Contributed by Peter Simpson

Guaranteed DNA
A few years ago a false report of a "Haplogroup," issued by another company, led two major US newspapers to report that an accountant from Florida was a genetic descendent of Genghis Khan. Family Tree DNA subsequently tested his SNP and we proved that he was, in fact, of a different haplogroup.  After retractions by both newspapers we launched our SNP assurance program. http://www.familytreedna.com/SNP_assurance.html  which guarantees correct placement in the phylogenetic tree through SNP testing.
- Contributed by Bennett Greenspan - President, Family Tree DNA

Confirming the Paper Trail
A member of one of my surname projects is African American, but he was predicted by the testing company to be in Haplogroup I1b, which suggests that his paternal line came from Europe, rather than Africa. The participant had traced his ancestry through traditional research back to a slave who lived in the mid-1800s, and he wondered if the slave might have been the son of one of the family who owned him. However, a descendant of the owner’s family in the project did not match his STR profile. SNP testing was ordered and the participant was found to be in Haplogroup B, which is found almost exclusively in sub-Saharan Africa. Now the participant knows the real origin of his paternal line.
- Contributed by Whit Athey

Many thanks and appreciation to the contributors of this compilation!
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Compiled on: 01/16/2008