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expressed in his offspring, while the copy from mothers is silenced. “That means what your father passes on to you is of massive significance,” Curley said. He is studying the gene in mice, but a form of Peg3 occurs in humans, too. So anything he discovers in his mice is likely to be true in us as well. To help me understand what the gene does, Curley started with a short lecture on mouse sex. Virgin male mice, he explained, begin with a trial-and-error mating strategy: they pursue any female they can, whether or not the female is in estrus, ready to reproduce. The males usually manage to mate, and then their troubles are over. Once they’ve mated, they develop the ability to detect, by smell, which females are in estrus. Curley wanted to know how Peg3 might be involved in this behavior, and so he used a lab trick to inactivate, or “knock out,” the Peg3 gene in some of his mice. The knockout mice were unable to detect when females were in estrus, even after they’d mated. They continued to try mating with females who were not ready to reproduce. After a while, the directionless males gave up. So now Curley knew that Peg3 is essential for the development of proper behavior regarding sex and mating in males.
    Curley then looked at females. There he discovered that knocking out Peg3 had a very different effect. Knocking out Peg3 in female mice doesn’t affect mating, the way it does in males. Instead, females whose Peg3 gene is knocked out become poor caretakers. They don’t eat as much as they should early in their pregnancies. After birth, they are supposed to lick their pups, nurse them, eat the placenta (a source of nourishment), and build a nest. Females whose Peg3 has been knocked out do those things much less frequently than normal mice.
    To sum all this up, Peg3 affects how well a father’s male pups will mate and how well his female pups will care for their offspring. The mating ability of his sons and the nurturing qualities of his daughters will both affect the health of his grandpups. Once again, we have an effect that extends from males not only to their children but also to their grandchildren. It’s reasonable to expect that the human version of Peg3 has similar effects in human males and their offspring. That’s not the same as proving the connection, but it gives Curley and others confidence to look for a similar phenomenon in humans.
    We can’t put people in cages, tag them with colorful jewelry, and allow them to mate. Unlike finches, who had no say in which leg band they got, human males might object to being made unattractive for the sake of an experiment. Nor would they appreciate being manipulated in a way that could harm their children. But mice, finches, and humans are enough alike that’s what true in them is often true in us. Even though these findings haven’t been confirmed in humans, it might be wise for men who are about to become fathers to think about their health and about what they should be eating even before their wives or partners become pregnant. This would be good advice for fathers even if it doesn’t have any beneficial effects on their children. And it would be an even better idea if it does.

 
    TWO
    Conception : The Genetic Tug-of-War
    Years ago, when I had just started work as a science reporter at the Associated Press, I happened to get into a cab with a biology student from MIT. We were both on the way to a conference on cancer in Houston, and he was nervously preparing to make one of his first scientific presentations at a national meeting—on a study of the Y chromosome, the vehicle for fathers’ genetic contributions to their kids. Our paths have crossed several times over the years, and I’ve watched his career soar. That student, David C. Page, is now the director of the distinguished Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, affiliated with MIT. And he’s still pursuing the questions he was interested in when we shared