All men and only men have a Y chromosome. This biological fact allows us to trace back in time a direct, largely unchanged genetic line of inheritance from father to son.
The second thing that makes the Y chromosome unique is that the information carried on Y chromosomes is inherited largely intact over time. Unlike other chromosomes, the genetic material on the Y chromosome is not mixed with each new generation. The information carried on the Y chromosome travels from father to son as a nearly exact copy of itself.
Occasionally, during the DNA copying process, small changes or mutations occur, and it is these mutational differences that allow us to distinguish the Y chromosome of an individual from his ancestor's. Thus, an actual genetic record of the male line going back through time exists -- as clear a marker of paternal heritage as a father's family name.
A tangible timekeeper of history, the Y chromosome allows us to trace human evolution, track migration patterns and relatedness in groups of people, and answer paternity questions going back generations. Through testing of the Y chromosome, we begin to unravel some fascinating stories about our own origins.