theatlantic | But
chimeras are not just oddities. You surely know one. In pregnant women,
fetal stem cells can cross the placenta to enter the mother’s
bloodstream, where they may persist for years. If Mom gets pregnant
again, the stem cells of her firstborn, still circulating in her blood,
can cross the placenta in the other direction, commingling with those of
the younger sibling. Heredity can thus flow “upstream,” from child to
parent—and then over and down to future siblings.
The
genome, Zimmer goes on to reveal, eludes tidy boundaries too. Forget
the notion that your genome is just the DNA in your chromosomes. We have
another genome, small but vital, in our cells’ mitochondria—the tiny
powerhouses that supply energy to the cell. Though the mitochondrial
genes are few, damage to them can lead to disorders of the brain,
muscles, internal organs, sensory systems, and more. At fertilization,
an embryo receives both chromosomes and mitochondria from the egg, and
only chromosomes from the sperm. Mitochondrial heredity thus flows
strictly through the maternal line; every boy is an evolutionary dead
end, as far as mitochondria are concerned.
Beyond the genome are
more surprises. Schoolchildren learn that Darwin’s predecessor, the
great French naturalist Jean-Baptiste Lamarck, got heredity wrong when he suggested that traits acquired through
experience—like the giraffe’s neck, elongated by straining and
stretching to reach higher, perhaps tenderer, leaves—could be passed
down. The biologist August Weismann famously gave the lie to such
theories, which collectively are known as “soft” heredity. If Lamarckism
were true, he said, chopping the tail off mice and breeding them,
generation after generation, should eventually produce tailless mice. It
didn’t. Lamarck wasn’t lurking in the details.
Recent
research, however, is giving Lamarck a measure of redemption. A subtle
regulatory system has been shown to silence or mute the effects of genes
without changing the DNA itself. Environmental stresses such as heat,
salt, toxins, and infection can trigger so-called epigenetic responses,
turning genes on and off to stimulate or restrict growth, initiate
immune reactions, and much more. These alterations in gene activity,
which are reversible, can be passed down to offspring. They are
hitchhikers on the chromosomes, riding along for a while, but able to
hop on and off. Harnessing epigenetics, some speculate, could enable us
to create Lamarckian crops, which would adapt to a disease in one or two
generations and then pass the acquired resistance down to their
offspring. If the disease left the area, so would the resistance.
All
of these heredities—chromosomal, mitochondrial, epigenetic—still don’t
add up to your entire you. Not even close. Every one of us carries a
unique flora of hundreds if not thousands of microbes, each with its own
genome, without which we cannot feel healthy—cannot be “us.” These too
can be passed down from parent to child—but may also move from child to
adult, child to child, stranger to stranger. Always a willing volunteer,
Zimmer allowed a researcher to sample the microbes living in his
belly-button lint. Zimmer’s “navelome” included 53 species of bacteria.
One microbe had been known, until then, only from the Mariana Trench.
“You, my friend,” the scientist said, “are a wonderland.” Indeed, we all
are.
With this in mind,
reconsider the ongoing effort to engineer heredity. The motto of the
Second International Eugenics Congress, in 1921, was “Eugenics is the
self-direction of human evolution.” Since then, controlling heredity has
become technically much easier and philosophically more complicated.
When, in the 1970s, the first genetic engineering made medical gene
therapy feasible, many of its pioneers urged caution, lest some
government try to create a genetic Fourth Reich. In particular, two
taboos seemed commonsense: no enhancement, only therapy (thou shalt not
create a master race); and no alterations in germ-line tissues, only in
somatic cells (thou shalt not make heritable modifications).
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