Here’s something you probably learned once in a biology class, more or less. There’s this molecule called DNA. It contains a long code that created you and is unique to you. And faithful copies of the code live inside the nucleus of every one of the trillions of cells in your body.
In a later class you may have learned a few exceptions to that “faithful copies” bit. Sometimes, especially during development, when cells are dividing into more cells, a mutation pops up in the DNA of a daughter cell. This makes the daughter cell and all of its progeny genetically distinct. The phenomenon is called ‘somatic mosaicism’, and it tends to happen in sperm cells, egg cells, immune cells, and cancer cells. But it’s pretty infrequent and, for most healthy people, inconsequential.
That’s what the textbooks say, anyway, and it’s also a common assumption in medical research. For instance, genetic studies of living people almost always collect DNA from blood draws or cheek swabs, even if investigating the tangled roots of, say, heart disease or diabetes or autism. The assumption is that whatever genetic blips show up in blood or saliva will recapitulate what’s in the (far less accessible) cells of the heart, pancreas, or brain.
Two recent reports suggest that somatic mosaicism is far more common than anybody ever realized — and that might be a good thing.
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