Strange twists and turns in the process of hered­ity

The Washington Post Sunday - - BOOK WORLD - RE­VIEW BY JERRY A. COYNE

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, as in the anal­y­sis of his own pedi­gree and genome, mat­ters likely to in­ter­est other Zim­mers more than the reader.

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 — who, via an­cient hy­bridiza­tion with our an­ces­tors, con­tributed about 2 per­cent of the genomes of mod­ern non-Africans — 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. (As Zim­mer cor­rectly ob­serves, these en­vi­ron­men­tally in­duced traits al­ways dis­ap­pear after a few gen­er­a­tions, so they can’t be the ba­sis for long-term “non-Dar­winian” evolution.)

Other fas­ci­nat­ing but un­con­ven­tional forms of in­her­i­tance in­clude the “hor­i­zon­tal” trans­mis­sion of genes be­tween wildly dif­fer­ent species (aphid DNA, for in­stance, con­tains genes pur­loined from fungi) and the in­her­i­tance of the bac­te­rial “mi­cro­biome” be­tween peo­ple through con­tact or birth, con­fer­ring sim­i­lar groups of bac­te­ria on re­lated peo­ple. (Zim­mer notes that he car­ries 53 species of bac­te­ria in his navel alone.) We’re only now be­gin­ning to learn how im­por­tant these mi­cro­bial fel­low travelers are for our health.

To me, the most en­gross­ing part of the book de­scribes hu­man “chimeras”: peo­ple whose bod­ies con­tain dif­fer­ent genomes. This can, for ex­am­ple, re­sult from the move­ment of cells back and forth across the pla­centa, so that a mother can be col­o­nized by groups of cells de­rived from off­spring born decades pre­vi­ously. Con­versely, a child can har­bor cells from its mother. Cells can also move be­tween twins in utero — or even, via the mother, be­tween off­spring from dif­fer­ent preg­nan­cies. One study showed that 13 per­cent of young girls had Y-chro­mo­some-car­ry­ing (i.e., male) cells in their blood, their source be­ing older broth­ers whose cells had col­o­nized Mom and then moved back through the pla­centa to in­habit Lit­tle Sis­ter.

But it gets even more twisted: Oc­ca­sion­ally a chimeric in­di­vid­ual is formed when the em­bryos of two fra­ter­nal twins fuse early in de­vel­op­ment. Such chimeras can have ma­jor por­tions of their bod­ies car­ry­ing dif­fer­ent genomes, some cells de­rived from Twin A and oth­ers from Twin B. This has pro­duced bizarre pa­ter­nity tests ap­par­ently show­ing that a woman is not the mother of her own child. This hap­pens when chimerism re­sults in a woman’s blood cells be­ing de­rived from Twin A (blood cells are used to de­ter­mine the mother’s ge­netic iden­tity), while her re­pro­duc­tive cells — the ones that pro­duced the egg that be­came Ju­nior — are de­rived from Twin B. Such ge­netic facts seem al­most alien.

Ad­mit­tedly, most of us are not ge­netic chimeras, but this tale il­lus­trates two things: the weird­ness of hered­ity — it’s much more than whether you in­her­ited your un­cle’s blue eyes — and the power of mod­ern science and tech­nol­ogy to un­ravel these ge­netic puz­zles. The message here is an im­por­tant one: Chimeras not­with­stand­ing, ge­netic tech­nolo­gies have well and truly ar­rived and are poised to have a huge im­pact on our lives. Ge­net­i­cally mod­i­fied crops are bur­geon­ing. What does that mean for us and for the en­vi­ron­ment? The Hu­man Genome Project — that com­plete cat­a­logue of the more than 3 bil­lion A’s, T’s, G’s and C’s that con­sti­tute our genome — cost around $3 bil­lion in the 1990s. To­day, it will run you just $1,000 to se­quence your en­tire genome; for the price of a year’s worth of cof­fee at Star­bucks, you can take a voy­age of dis­cov­ery into your ge­netic heart, learn­ing who you are and where you came from. And soon we’ll be able to ma­nip­u­late a fer­til­ized hu­man egg, chang­ing any genes we want — not just those caus­ing dis­ease but those al­ter­ing be­hav­ior, ap­pear­ance and in­tel­li­gence.

We need to know about these de­vel­op­ments. Can I load the Men­delian dice when I have a child, en­sur­ing that it re­ally does get Un­cle Frank’s blue eyes? Should we al­low peo­ple to cre­ate “de­signer ba­bies” with higher IQs? How likely am I to get the cancer that killed my grand­mother? And if I do get it, what can I do to get rid of it?

Zim­mer’s book is an ex­cel­lent way to get up to speed in these ar­eas, but be aware that there are a cou­ple of re­cent com­peti­tors that give much the same in­for­ma­tion. These in­clude Sid­dhartha Mukherjee’s su­perbly writ­ten chronol­ogy “The Gene: An In­ti­mate His­tory,” and James Wat­son’s lav­ishly il­lus­trated (and largely first-hand) ac­count, “DNA: The Story of the Ge­netic Rev­o­lu­tion.” Read at least one of the three, be­cause this is im­por­tant stuff: If the science doesn’t mat­ter to you now, it will soon.

Jerry A. Coyne is pro­fes­sor emer­i­tus in the de­part­ment of ecol­ogy and evolution at the Univer­sity of Chicago. He is the au­thor of “Spe­ci­a­tion” (with H. Allen Orr), “Why Evolution Is True” and “Faith vs. Fact: Why Science and Re­li­gion Are In­com­pat­i­ble.”


By Carl Zim­mer Dut­ton. 657 pp. $30

SHE HAS HER MOTHER’S LAUGH The Pow­ers, Perver­sions, and Po­ten­tial of Hered­ity

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