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.
Every person, male or female, has 22 matching pairs of chromosome—one
inherited from each parent—but the 23rd pair is different. This unmatched
pair, known as the X and Y sex chromosomes, determines whether we are male (XY)
or female (XX). A mother always provides a single X chromosome in her egg.
Inherit an X from your father and you will be a female, receive a copy of his Y
you will be male. And so the Y chromosome travels from father to son with each
successive generation of males.
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 reason is that when cells divide in preparation to make
sperm and egg, all 23 chromosome pairs line up to exchange random bits and
pieces of DNA with their matching partner before separating.
All chromosomes do this exchange of genetic material save the mismatched XY
pair. The Y is much shorter, and very little of its genetic information is
broken up in an exchange of DNA with the X chromosome. The information carried
on the Y chromosome travels from father to son as a nearly exact copy of
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. As we pull apart the Y
chromosome, we begin to unravel some fascinating stories about our own