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its mitochondria are not only vastly outnumbered but are deliberately destroyed. This is why,
although the fertilized egg contains nuclear DNA from both father and mother, all the mitochondria,
and so all the mitochondrial DNA, is from the mother.
The process is repeated generation after generation after generation. Nuclear DNA comes from
the father and mother, mDNA only from the mother. Consider your own mDNA for a moment. It is
powering your aerobic metabolism in every cell – from the cells in your retina which collect the
focused image from the page, to the muscles in your arm that turn the pages, to the cells that are
burning fuel to keep you warm. All these functions are controlled by your mDNA which, because of
its unusual inheritance, you have got only from your mother. Who got it from her mother. Who got it
from her mother and so on. At any time in the past, be it 100, 1,000, even 10,000 years ago, there
was only one woman alive at the time from whom you have inherited your mDNA. Even though I
have known this for years it still amazes me to think about it.
The combination of plenty of genetic variation with its matrilineal inheritance makes mDNA the
perfect guide to the human past. But it needs to be complemented, because it can tell only one side
of the story. Mitochondrial DNA can only tell the history of women. Very fortunately, there is a
piece of DNA which can do the same for men. This companion guide to our genetic history could
not be more different. This is the piece of DNA that is entirely male. It is the Y-chromosome.
Inside the nucleus of every human cell are a total of forty-six chromosomes. Forty-four out of
the forty-six carry on them the great majority of the 10,000 genes that build and run our bodies.
They include the blood-group, collagen and haemoglobin genes we have already met and many,
many more. They direct almost everything, from aspects of our physical appearance like eye and
hair colour, to our immune systems, to our innate psychological and emotional make-up. In
everybody, male and female, these forty-four chromosomes come in pairs and are inherited from
both parents, twenty-two from one, twenty-two from the other.
The other two chromosomes, called X and Y, are different in that they are not always inherited
from both parents. And not everybody has both of them. Females have two X-chromosomes and
men have one X-chromosome and one Y-chromosome. In the official notation of genetics, women
are XX and men are XY. However, despite what I have come to appreciate that most people
believe, the X-chromosome has nothing directly to do with sex. Women are not women because
they possess two X chromosomes – the truth is far more interesting. Women are female because
they don’t have a Y-chromosome. How can that be?
Looked at under the microscope, the X and Y chromosomes look quite different. Both are the
same shape, like tiny threads, but the X-chromosome is about five times as long. The differences
between X and Y don’t stop there. Thanks to the output from the Human Genome Project we now
have the DNA sequence for both chromosomes. The larger X-chromosome is very like the other
forty-four chromosomes. It carries about 1,000 genes which control a range of different cellular
activities. The Y-chromosome, on the other hand, is a genetic wreck with only twenty-seven genes
that appear to be working properly. The rest of the chromosome is made up of long stretches of socalled
‘junk’ DNA. This is DNA that, unlike genes that do things, has no known function. It is just
there. The evolutionary implications for this tremendous difference between X- and Y-
chromosomes are fascinating, but not especially relevant here. What does matter is that just one of
the twenty-seven active genes on the Y-chromosome, the sex gene, is what makes males.
For the first six weeks of life, there is no visible difference between male and female embryos.