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At about that time, the sex gene on the Y-chromosome switches on. This sends a signal to a whole
series of other genes situated on other chromosomes, which, between them, actively divert
embryonic development away from female and towards male. Embryos that don’t have a Y-
chromosome just carry on along the normal female development pathway and are born girls. The
X-chromosome has nothing to do with it. Men truly are genetically modified women.
This mechanism for deciding sex which humans have inherited from their distant mammalian
ancestors creates the second of our guides to our genetic origins. Men carry both an X- and a Y-
chromosome in all of their cells – except mature sperm. Sperm occur in two different genetic
forms, indistinguishable under the microscope and in their swimming capabilities. Stem cells in the
male testis are dividing furiously to keep up the supply of sperm and like the other cells in the body
have the XY combination of sex chromosomes. At the final division, the cell divides one last time
but the resulting sperm only get one of the sex chromosomes, not both. Half the sperm receive an X-
chromosome from this division while the other half get a Y-chromosome. The sex of the child
entirely depends on which sort of sperm wins the race to the egg. If it’s got an X-chromosome then
the egg, which already has one X-chromosome, becomes XX after fertilization, develops as a
female embryo and is born a girl. If, on the other hand, the winning sperm contains a Y-
chromosome, the fertilized egg becomes XY and develops into a boy. The simple conclusion is
this: Y-chromosomes get passed down the male line from father to son.
Looking backwards, if you are a man, you got your Y-chromosome from your father, who got it
from his father. Who got it from his father. Sounds familiar? It is the mirror image of the inheritance
pattern for mitochondrial DNA. The Y-chromosome is the perfect complement to mDNA, telling
the history of men. But does it have enough genetic variability to be practically useful? It took a
very long time to find any mutations at all on the Y-chromosome. For those scientists involved, and
thankfully I wasn’t one of them, it was a frustrating few years. In one of the first studies looking for
diversity among human Y-chromosomes, 14,000 bases were sequenced from twelve men from
widely scattered geographical localities. Only a single mutation was discovered. Another lab
sequenced the same 700-base segment from the Y-chromosomes of thirty-eight different men and
didn’t find a single mutation in any of them. At long last, and helped by an ingenious technique for
finding the elusive mutations, the Y-chromosome began to show its genetic jewels. Slowly, slowly,
mutations that had changed one DNA base to another were teased out of the otherwise barren desert
of uniformity.
With these two pieces of DNA we have the perfect companions for our exploration of the
genetic past. One follows the female line, the other tracks the male genealogy. What could be
better? They had been my guides in Polynesia and in Europe and I knew them well. Among their
many qualities is that they both group people into clans. When my colleagues and I had been trying
to make sense of the mDNA results from Europe in the early 1990s, we noticed that the 800 or so
samples from volunteers from all over Europe fell into seven quite distinct groups based on their
mDNA sequences.
Unlike the chromosomes in the cell nucleus, which are straightforward linear strings of DNA,
mitochondrial DNA is formed into a circle, which is a hangover from when the mitochondria
themselves were free-living bacteria. The human mitochondrial DNA circle is exactly 16,589 DNA
bases in length, but fortunately it is unnecessary to read the entire sequence. Most of the
mitochondrial DNA circle is taken up with genes that code for the enzymes involved in aerobic