For years, genes have been considered the one and only way
biological traits could be passed down through generations
Now research into epigenetics has shown that environmental
factors affect characteristics of organisms. These changes
are sometimes passed on to the offspring.
Increasingly, biologists are finding that non-genetic variation
acquired during the life of an organism can sometimes be passed
on to offspring—a phenomenon known as epigenetic inheritance.
An article forthcoming in the July issue of The Quarterly
Review of Biology lists over 100 well-documented cases
of epigenetic inheritance between generations of organisms,
and suggests that non-DNA inheritance happens much more often
than scientists previously thought.
Biologists have suspected for years that some kind of epigenetic
inheritance occurs at the cellular level. The different kinds
of cells in our bodies provide an example. Skin cells and
brain cells have different forms and functions, despite having
exactly the same DNA. There must be mechanisms—other
than DNA—that make sure skin cells stay skin cells when
Only recently, however, have researchers begun to find molecular
evidence of non-DNA inheritance between organisms as well
as between cells. The main question now is: How often does
"The analysis of these data shows that epigenetic inheritance
is ubiquitous …," write Eva Jablonka and Gal Raz,
both of Tel-Aviv University in Israel. Their article outlines
inherited epigenetic variation in bacteria, protists, fungi,
plants, and animals.
These findings "represent the tip of a very large iceberg,"
the authors say.
For example, Jablonka and Raz cite a study finding that when
fruit flies are exposed to certain chemicals, at least 13
generations of their descendants are born with bristly outgrowths
on their eyes. Another study found that exposing a pregnant
rat to a chemical that alters reproductive hormones leads
to generations of sick offspring. Yet another study shows
higher rates of heart disease and diabetes in the children
and grandchildren of people who were malnourished in adolescence.
In these cases, as well as the rest of the cases Jablonka
and Raz cite, the source of the variation in subsequent generations
was not DNA. Rather, the new traits were carried on through
There are four known mechanisms for epigenetic inheritance.
According to Jablonka and Raz, the best understood of these
is "DNA methylation." Methyls, small chemical groups
within cells, latch on to certain areas along the DNA strand.
The methyls serve as a kind of switch that renders genes active
By turning genes on and off, methyls can have a profound
impact on the form and function of cells and organisms, without
changing the underlying DNA. If the normal pattern of methyls
is altered—by a chemical agent, for example—that
new pattern can be passed to future generations.
The result, as in the case of the pregnant rats, can be dramatic
and stick around for generations, despite the fact that underlying
DNA remains unchanged.
New evidence for epigenetic inheritance has profound implications
for the study of evolution, Jablonka and Raz say.
"Incorporating epigenetic inheritance into evolutionary
theory extends the scope of evolutionary thinking and leads
to notions of heredity and evolution that incorporate development,"
This is a vindication of sorts for 18th century naturalist
Jean Baptiste Lamarck. Lamarck, whose writings on evolution
predated Charles Darwin's, believed that evolution was driven
in part by the inheritance of acquired traits. His classic
example was the giraffe. Giraffe ancestors, Lamarck surmised,
reached with their necks to munch leaves high in trees. The
reaching caused their necks to become slightly longer—a
trait that was passed on to descendants. Generation after
generation inherited slightly longer necks, and the result
is what we see in giraffes today.
With the advent of Mendelian genetics and the later discovery
of DNA, Lamarck's ideas fell out of favor entirely. Research
on epigenetics, while yet to uncover anything as dramatic
as Lamarck's giraffes, does suggest that acquired traits can
be heritable, and that Lamarck was not so wrong after all.