Read More: Nat Geo
No, seven percent of Kelly’s DNA is not mutated after his year in space, which makes sense if you keep in mind that humans and chimps have genetic sequences that differ by less than 2 percent, and individual humans—even completely unrelated strangers—differ by about 0.1 percent. Here’s how this works. Genetic sequences are like strings of letters arranged just so, and they are in charge of producing proteins. Mutate the wrong letter, or sequence of letters, and it can be mildly annoying, like our favorite ducking autocorrect, or it can be extremely bad news, like the mutations that allow tumors to proliferate. Most mutations, however, go unnoticed (the one that confer superpowers are a different story). Expression levels, on the other hand, reflect whether genes are turned on or turned off. Within each of us, most of our cells are otherwise genetically identical, but their genes are expressed at different levels. It’s those patterns of expression that produce hearts, brains, eyeballs, and other things, kind of like using the same set of ingredients to cook up vastly different dishes.