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Mom's Genes or Dad's? Map Can Tell.

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By Rick Weiss
Washington Post Staff Writer
Tuesday, September 4, 2007

Scientists have for the first time determined the order of virtually every letter of DNA code in an individual, offering an unprecedented readout of the separate genetic contributions made by that person's mother and father.

By providing a detailed look at maternal and paternal DNA strands, rather than the blended composite that was yielded by the 2001 Human Genome Project, the work offers the clearest snapshot yet of just how different those two contributions can be. Assuming the newly decoded sequence is typical, as scientists presume it is, there are five times as many differences between individuals' DNA as was previously thought.

Of more practical import, the ability to create such a detailed genetic profile with relative ease suggests that it may not be long before people of ordinary means will be able to have their complete DNA codes spelled out, scientists said. That could tell a lot about a person's health risks, because such a profile would include not only the few genes that significantly increase the likelihood of getting certain diseases but also the many "lesser" genes that pose modest risks individually but that together have the bulk of impact on health.

For better or worse, the advance also stands to bring science to the pastime of guessing which parent deserves blame or credit for passing along certain traits.

"This is the ultimate form of genealogy. You'll have incredible information about yourself," said Stephen Scherer of the Hospital for Sick Children in Toronto, who was part of the multimillion-dollar project described in today's issue of the journal PLoS Biology. "I wouldn't be surprised if Internet-based browsers pop up before long that allow you to compare your genome to others."

The genetic sequence that was unveiled in all its naked detail belongs to J. Craig Venter, the Maryland scientist who led the project at the J. Craig Venter Institute in Rockville. It carries significantly more information than the two previously sequenced human genomes, released with great fanfare six years ago.

Those sequences -- one assembled by Venter and co-workers at Celera Genomics and the other by federally funded scientists -- were composites of several people. And although they were referred to as complete, they were in fact half-genomes -- or "haploid" -- containing a mom-and-pop mosaic of the 3 billion DNA letters found on just one set of the 23 chromosomes paired in every cell.

Not emphasized in 2001 was the fact that people have in their cells two versions of each of those 23 chromosomes, one from each parent -- a "diploid" genome. And increasingly scientists are finding that the difference between being healthy and being sick has a lot to do with how those genomes interact.

From Dad, a person may inherit a version of a gene that predisposes her to a disease, but from Mom she may inherit a protective version -- or an entirely different gene elsewhere on the genome that counteracts Dad's contribution.

"I might want to know: Do I have an additive risk from the genomes from both my parents, or did I get some helpful ones from her that counteract the ones from him?" Venter said.

Sorting out those details has been daunting.

"It's very easy to start mixing up the readouts from each parent because they are so similar," said Samuel Levy, who led the research with Venter. But new sequencing technologies and computational methods allow scientists to chop a person's DNA into pieces and reassemble the maternal and paternal segments independently.


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