Are We Chimeras?

Chimera of Arezzo

Wikipedia

We may all be chimeras! Not the mythical sort depicted above, but the sort defined by scientists as "a single organism . . . composed of two or more . . . populations of genetically distinct cells" (Wikipedia). But why do I say we may all be chimeras? An article by Carl Zimmer titled "DNA Double Take" (NYT, September 16, 2013) informs us that "scientists are discovering that -- to a surprising degree -- we contain genetic multitudes." Walt Whitman would be proud! But I find this fact a bit scary, especially when I read the following:

One woman discovered she was a chimera as late as age 52. In need of a kidney transplant, she was tested so that she might find a match. The results indicated that she was not the mother of two of her three biological children. It turned out that she had originated from two genomes. One genome gave rise to her blood and some of her eggs; other eggs carried a separate genome.

Apparently, she and her nonidentical twin fetus had combined early in the womb. I wonder how doctors decided which genome the woman really was. The one making up the larger proportion? Suppose one's fat cells were a different genome -- lose weight and become a different person! Chimeras, by the way, don't result solely from two different genomes combining during early fetal development:

Women can also gain genomes from their children. After a baby is born, it may leave some fetal cells behind in its mother's body, where they can travel to different organs and be absorbed into those tissues. "It's pretty likely that any woman who has been pregnant is a chimera," Dr. Randolph said.

Evidence? Consider:

As scientists begin to search for chimeras systematically -- rather than waiting for them to turn up in puzzling medical tests -- they're finding them 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.

Foreign genomes get into our bodies in other ways, too, and can be a factor in various diseases, or even if not malignant -- and most apparently are not -- can distort the results of forensic DNA tests used as courtroom evidence, but some foreign genomes can also even be helpful. The article touches on all of this.