- Revised 10/06/2015
To view a wonderful SNP video from basics to advanced go to Who Do you think you are presentation
See at this site: SNPs are considered “once in a lifetime,” or maybe better stated, “once in the lifetime of mankind” type of events, known as a UEP, Unique Event Polymorphism, where STRs happen “all the time,” in every haplogroup.
D Phelps comment: A numeric position on the DNA chain that mutates is then called a SNP. Once found it is given a new number prefixed by a letter code defining the researcher and is tested afterwards by the new identification.. By the way, IMO, it is possible to find a terminal snp (happens every 150 years or so) even within family branches in genealogical time. In theory therefore, you, yourself, could have a new personal terminal snp, which would then be found in all your descendants “forever”. That is what had to happen in the first person who defined a new sub-haplogroup such as e1a1 and in all the downstream subhaplogroups. This concept is the basis for my hope of the two additional Phelps for doing a big y: a family branch SNP.
A major topic of confusion and frustration are the implications of the term “terminal SNP” A starting point of discussion may well be in this definition here at ISOGG A person has many inherited SNPs that together create a unique DNA pattern for that individual. Snips clarify the branching of a tree-separation, identified by as subclades or subhaplogroups . The oldest snps define deep ancestry. A terminal SNP is the “defining” SNP of the latest subclade known by current research. It should be unique (UEP) and constant in time. Thus with continuing Big Y tests we move closer and closer to the present time - or the "final" terminal SNP, usually called a family or private SNP.
From the FTDNA Forum posts:
A terminal SNP is simply the defining SNP of the latest subclade known by current research.
The Haplotree is a growing thing. You may have one terminal SNP today and when your branch of the tree grows you'll have another terminal SNP.
Can you have more than one terminal SNP? Yes, because many SNPs may be at the same tree level--they are considered phylogenetically "equivalent." Of course, that is only true until a counterexample is found that splits them out. [Comment: thus some use 150 years per equivalent SNP allows an estimate of time back to the subclade breakout.]
Your terminal SNP is the SNP at the end of your Haplotree branch. That SNP may have more than one name, so yes you can have more that one name for your terminal SNP. Whose Haplotree are you using?. There are several trees that have longer branches than the tree FTDNA uses so FTNDA may tell you XYZ is your terminal SNP and another, bigger tree may say that ABC23 is your terminal SNP.
From the Family Tree DNA Learning Center,
Questions and Answers about SNPs
The Big Y SNP test
Family SNPs : Are Y-chromosome DNA (Y-DNA) SNPs ever related to recent times and genealogy? Are they always for historic periods and more ancient ancestry? Y-chromosome DNA SNPs (single nucleotide polymorphisms) are not always from anthropological or even historic times. Some Y-chromosome DNA SNPs are restricted to a single family that is related in genealogical times (one to 15 generations). These are family SNPs or private SNPs. Other SNPs are slightly older. They represent paternal lineage groups that predate the adoption of a family surname. These are semi-private SNPs.
Currently, family SNPs and semi-private SNPs are not common.
SNP Testing review by ISOGG (including a video)