Strange twists and turns in the process of heredity
Why do children look like their parents and siblings, but still differ from one another? This problem, sitting right at the heart of biology, was solved in part by Gregor Mendel and his early-20th-century heirs. Still, we had to wait until the advent of DNA sequencing in the 1970s before we could penetrate these mysteries in detailed, mechanistic ways.
Carl Zimmer's new book, “She Has Her Mother's Laugh,” lays out what we have learned. His approach is dangerously encyclopedic — my copy of the book weighs several pounds — because he chooses (rightly, in my view) to combine the history of the field with a detailed account of current developments. It's an ambitious undertaking, one that requires a light authorial touch to avoid a result that is dense, and turgid, and boring. Zimmer, a prominent science journalist who writes regularly for the New York Times, does a good job of avoiding the encyclopedia trap, larding his account with plenty of colorful stories. At times, however, he does get mired in overly long stories while trying to give the book a folksy feel.
On the other hand, Zimmer is not just a journalist but a co-author of a highly regarded textbook on evolutionary biology, meaning that he really understands what he is writing about. The strength of the book, then, is its combination of accuracy, journalistic clarity and scientific authority.
And there's plenty of science. Mendel and Charles Darwin appear, of course; the dark history of eugenics stalks the pages; our new ability to explore the tree of life by comparing DNA sequences gets airplay; the hunt for genetic errors that cause disease occupies several chapters; CRISPR and other cutting-edge technologies that can (and will) allow us to edit out these errors are carefully explained; the story of human migration throughout the
world is reconstructed using our DNA; and Neanderthals get some headlines of their own.
The book really takes off when it hews to Zimmer's main theme: that heredity extends far beyond the kind of vertical parent-to-offspring transmission of DNA we normally think of as “genetics.” As Zimmer notes, “We cannot understand the natural world with a simplistic notion of genetic heredity.” And so we enter the world of developmental biology, learning how a single fertilized egg can produce more than 200 different cell types in the adult body, all containing exactly the same genes but using them differently. And each cell, after acquiring its identity, passes on its unique traits when it divides. We learn how the external environment can produce an unconventional form of heredity, as when rats learn to fear certain odors experimentally associated with an electric shock — a fear that can be inherited by offspring who never experienced the shocks.
Other fascinating but unconventional forms of inheritance include the “horizontal” transmission of genes between wildly different species, and the inheritance of the bacterial “microbiome” between people through contact or birth. We're only now beginning to learn how important these microbial fellow travelers are for our health.