For decades, we as a culture have referred to a person’s genome as a singular thing — a set of DNA that exists in each cell and is unique to each person. You are a person and you have one genome.
But the New York Times this week brought to light mounting evidence that that’s not necessarily true. It introduced to the masses two terms that have been gaining acceptance among scientists over the past several years: chimerism and mosaicism.
Both reflect the notion that an individual can have more than one genome. A twin, for example, might acquire some of the other twin’s blood cells in the womb, and end up with part of his genome. A woman can apparently gain genomes from fetal cells left behind after she gives birth to a baby. People who have bone marrow transplants, too, have been shown in studies to end up with some of their donors’ genomes.
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What does this finding mean for medicine? The U.S. government undertook the $3 billion Human Genome Project more than two decades ago to improve DNA sequencing, which would help scientists better understand disease. Since then, a number of diagnostic tests and gene therapies have been developed for cancers and rare diseases based on the idea that a person’s genome represents his genetic makeup.
Now, according to Carl Zimmer in the Times:
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.
In other words, the complicated science of the human genome just got more complicated. Maybe, after all, a cheek swab won’t reveal as much about a person’s health as we once thought.
