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Of genes and motherhood: Even when our children resemble us, the outcome can be very different

I am the dark-haired, brown-eyed mother of two blond, blue-eyed children. In airports and parks, I expect to be mistaken for their nanny.

My abdominal scars can attest that they are my biological children and were built using some of "my" genes.

But we are not Mendel's peas, and the more we learn about genomics, the less pea-like we seem.

Even when we each start carrying around our personal genome disks in our pockets, our data will differ from our children's in thousands of ways. And even where our genes look identical to theirs, our bodies and minds could well differ, influenced by many other factors, including the portions of our DNA that don't code for genes, our environment, and our behavior.

So our changeling children can help remind us that our own lives and health will remain - for the most part - beyond our powers to predict. And our children's lives, with their mishmash of father's and mother's lineage, so much the more so.

"Genetics is not destiny," said Dana Waring of the Personal Genetics Education Project, a year-old effort funded by a Harvard genetics lab to inform and engage the public in thinking about the dawning personal genomic era. "The more we learn about genetics, the more complex and the more layered the story becomes."

It used to seem simpler. More than a century ago, Mendel wrote about genes as discrete traits: the offspring of a purple-flowering pea plant and a white-flowering pea plant would bloom purple or white in fairly predictable numbers, not lavender. Remember the little boxes from high school biology class with the capital B's for dominant brown eyes and the small b's for recessive blue eyes?

Some traits really are that simple - single-gene diseases such as Huntington's. And actually, eye color and hair color are passed on relatively simply as well - it's just that my children happened to inherit my husband's.

Researchers have long tended to focus on such simpler traits because they are easier to investigate and do explain a few serious diseases.

But that old Mendelian model is increasingly giving way now, both in research and in the public mind, to a more complex understanding of genetics, said Dr. David Altshuler, a Massachusetts General Hospital geneticist.

Genetic research has finally developed the tools to begin cracking the complex diseases that most hurt human health - cancer, diabetes, heart failure.

The great majority of things we care about passing on, and developing in ourselves, are complex traits as well, affected by multiple genes and the environment as well. Intelligence, temperament, even something as seemingly simple as height.

Earlier this month, researchers from MIT, Children's Hospital Boston, and elsewhere announced that they had pinpointed the first gene that was definitely, consistently associated with height in the general population. How much of a difference did it make? About one centimeter, or one-third of an inch, if you inherited two copies of the key variant.

The researchers figured that the gene, nailed down in a giant study of some 35,000 people, accounted for just 0.3percent of the total variation in height in humans. So how many genes do you think are involved in a more subtle characteristic, like the hand-eye coordination of athletic talent or facility with language? Hundreds? Thousands? Mixed in with millions of minutes in school and home environments?

You do see mini-me's here and there, children unmistakably all-but-cloned from a parent. And on average, your child will be much more like you than someone else, Altshuler said. But there is a multitude of genetic variants that influence who you are, and another multitude that influence your partner, so your kids end up a unique mixture that differs dramatically from each of you.

More genomic information may well cast some light on our chances for certain diseases and characteristics, Waring said. But - unsatisfying as this may be to many people - it will offer little black-and-white foreknowledge of future disease or a child's future brilliance. Mainly, it will "illuminate more gray areas," she said.

So if your children are not a visible repositories of your genes, what makes them yours?

Perhaps: Holding. Feeding. Diaper-changing. Letting them own you.

You may choose to have a child in hopes of replicating yourself, said Dr. Isaac Kohane, a Harvard Medical School computer scientist, genomics researcher and seasoned father of three.

But those "selfish genes," he pointed out, "are not the ones woken up at 2 a.m."

Carey Goldberg can be reached at goldberg@globe.com.

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