Here are some practical points regarding haplogroups and haplotrees that I've gleaned from working with the Petty Project at FTDNA over the past year or so.
The first thing to recognize about haplogroups, is that
( ( (a) each of us has an essentially unique genetic family tree, represented as a tree-structured sequence of haplogroups – called a haplotree – which is entirely analogous to a physical family tree.
Although a person’s haplotree consists of a fixed sequence of haplogroups,
b) (b) it takes specific genetic (SNP) testing, of the person as well as a number of his cousins, to uncover and identify them all.
Rather than a new generation being formed simply as a result of a man having a son, as is the case with physical generations,
c) (c) a new genetic generation is formed only when a mutation occurs in a man’s Y chromosome, and that mutation is then passed along to his son.
Another difference between a physical family tree and a haplotree is that
d) (d) a mutation does not necessarily occur in every “physical” generation; it can be quite variable, but on average, a new genetic generation is formed only about every 2 – 3 physical generations.
This is somewhat of an oversimplification, but basically,
e) (e) each mutation results in the formation of a new haplogroup.
It’s very important to recognize that
f) (f) once a mutation occurs, and a new haplogroup is created, those changes continue to be passed down to every subsequent (physical) generation.
Finally,
g) (g) the global male human haplotree is constantly growing and evolving, with new haplogroups being discovered every day, and because of this, downstream haplotrees are always subject to adjustment: unlike a grave marker, our haplotree is not etched in stone.
