Travel Reference
In-Depth Information
After two billion years the chromosomes that the ancestors of the mito-
chondria brought with them have lost most of their genes. Some of their
genes have been inserted into our own chromosomes, while others were
probably unneeded and are lost forever. Only thirty-four genes now remain
on the little ring-shaped piece of bacterial DNA that our mitochondria still
carry.
We inherit mitochondria and their chromosomes only from our moth-
ers. Our fathers have mitochondria too, because otherwise they would be
dead, but the mitochondria carried by the sperm are destroyed when the
sperm enters the egg. Only the females' mitochondrial chromosomes are
passed down from one generation to the next.
A brief history of dogs
This capsule history of the mitochondria now leads us into how we can
probe the histories of dogs and other domesticated animals.
Most of the genes on our chromosomes are mixed up through genetic
recombination and inherited from both parents, so that our children are
always dif erent from their parents. But our little snippets of mitochondrial
DNA are exactly like those of our mothers. The same pattern of inheri-
tance applies to dogs, which inherit their mitochondrial DNA in exactly
the same way. But over time, mutations will accumulate in mitochondrial
chromosomes, and they do so relatively rapidly. As a result of these accu-
mulating mutations, dog mitochondrial chromosomes dif er a little bit from
those of their wolf ancestors but can still be traced back to them.
As we saw in the fi rst chapter, comparisons of DNA sequences can help
to sort out the relationships among any group of living creatures. The more
closely related the DNA sequences of two organisms are, the closer they tend
to be on the family tree. Relying on these relationships, Peter Savolainen of
Stockholm's Royal Institute of Technology has built a family tree of mito-
chondrial DNA sequences from dogs and wolves around the world.
2
The tree that Savolainen and his colleagues obtained was a bit dii cult to
interpret, because there had only been time for a small number of mutational
