The famous double helix is a representation of the structure of DNA. For our purposes, to avoid unnecessary complication, let's look at the Y chromosome as a single string of beads, each one consisting of one of the four DNA molecules, abbreviated to A, C, T, and G.

The Y, which is puny compared to its counterpart chromosomes, has a mere 57 million-plus molecules (known as bases). They're strung together in a specific sequence and the constituent molecules are numbered (by scientists, not by nature) from 1 to 57227415.

This example is found in the sample of a member in the Cochran DNA Project. Position 11995317 of the Y chromosome has mutated from a C molecule (cytosine) to a T (thymine). It's a SNP mutation and has been named Y65968 by the genetic genealogy community for a very uninteresting reason.

Reminders

• When it comes to the Y, forget all you might have learned about DNA inheritance. The Y chromosome is inherited only by a son and only through his father. None of its components skip a generation and there are no dominant or recessive genes. Once a mutation occurs, it's passed down through every successive generation. Therefore, a living tester will have all the mutated markers accumulated by all his father-line ancestors. About every four generations, a new SNP hops onto the ever-rolling bus — all the way until the lineage dies out. Y-DNA SNPs represent a living biological archive of a specific lineage. The trick is to discover just when any one SNP came into the lineage.