Biologically speaking, what is an individual, pray tell?

The New York Times published a fascinating article yesterday, DNA Double Take, by Carl Zimmer, that calls into question the nearly universal belief that every cell in an individual's body contains the same DNA-- the DNA determining an individual's biological identity. This now appears to be a gross oversimplification.

[S]cientists are discovering that — to a surprising degree — we contain genetic multitudes. Not long ago, researchers had thought it was rare for the cells in a single healthy person to differ genetically in a significant way. But scientists are finding that it’s quite common for an individual to have multiple genomes. Some people, for example, have groups of cells with mutations that are not found in the rest of the body. Some have genomes that came from other people.

“There have been whispers in the matrix about this for years, even decades, but only in a very hypothetical sense,” said Alexander Urban, a geneticist at Stanford University. Even three years ago, suggesting that there was widespread genetic variation in a single body would have been met with skepticism, he said. “You would have just run against the wall.”

But a series of recent papers by Dr. Urban and others has demonstrated that those whispers were not just hypothetical. The variation in the genomes found in a single person is too large to be ignored. “We now know it’s there,” Dr. Urban said. “Now we’re mapping this new continent.”

Dr. James R. Lupski, a leading expert on the human genome at Baylor College of Medicine, wrote in a recent review in the journal Science that the existence of multiple genomes in an individual could have a tremendous impact on the practice of medicine. “It’s changed the way I think,” he said in an interview.

Scientists are finding links from multiple genomes to certain rare diseases, and now they’re beginning to investigate genetic variations to shed light on more common disorders.

Science’s changing view is also raising questions about how forensic scientists should use DNA evidence to identify people. It’s also posing challenges for genetic counselors, who can’t assume that the genetic information from one cell can tell them about the DNA throughout a person’s body.

Carrying more than one type of DNA is called "chimerism." It was previously considered rare, since evidence for it only occasionally thrust itself into visibility. Now that sequencing the human genome has become much less expensive to do, chimerism is becoming more visible. These weren't previously unknown-- Zimmer notes that in 1953, when a woman donated a pint of blood, some of which was Type O and some Type A, it was concluded that she'd acquired some of her blood from her twin brother, while in the womb. But now, scientists have begun to search systematically for chimeras-- and are finding that chimerism is not at all rare, and in fact occurs

in a remarkably high fraction of people. In 2012, Canadian scientists performed autopsies on the brains of 59 women. They found neurons with Y chromosomes in 63 percent of them. The neurons likely developed from cells originating in their sons.

In The International Journal of Cancer in August, Eugen Dhimolea of the Dana-Farber Cancer Institute in Boston and colleagues reported that male cells can also infiltrate breast tissue. When they looked for Y chromosomes in samples of breast tissue, they found it in 56 percent of the women they investigated.

Besides the implications for medicine, the discovery of the prevalence of chimerism is likely to complicate forensic science. Zimmer cites a case that arose last year, in which a saliva sample and sperm sample from the same suspect in a sexual assault case didn't match because the suspect had two sequences of DNA in his body.

I see the discovery of the prevalence of chimerism as further undermining the common, even cherished conception of the genome as a master blueprint that allows everyone to be neatly pigeonholed and offers the means for tinkering with and easily controlling desirable traits once the practical issues of genetic engineering have been worked out-- and, of course, bolsters the illusions people have about the biology of sex differences. Some people, of course, will be certain that acquiring neurons with Y-chromosomes will make a significant difference to the operation of women's brains following the birth of sons. I imagine there may even be a few neuroscientists willing to squander grant money in attempt to locate and prove such a difference. But such old, cliche-ridden projects will inevitably begin to look less and less interesting. Just the other day I read an article in Science reporting that the hottest, fastest-moving research of the day is on bacterial immune systems (and to tell the truth, until I read that article I hadn't even known bacteria have immune systems!)-- and that bacterial immune systems are being used to harness features to target the destruction of specific genes in human cells. Nothing about human biology is as simple as it once seemed. And I say: Glory hallelujah!