How DNA testing works

Want to find out more about yourself, your family or even your pet? There’s a DNA test for that. Spit in a tube. Let your dog chew a swab. Or pull out some of your cat’s fur. Then ship that sample off in the mail. A few weeks later, you can log into a testing company’s website to get a guide describing the traits suggested by that DNA.

The findings might predict someone’s hair color or suggest whether your genes will make you think the herb cilantro tastes like soap (though if you’ve tasted it, you probably already know what you think). The testing might go on to turn up relatives you didn’t know you had. If you got it for your dog, the test might say whether Fido has any German shepherd, corgi or poodle in its family tree. It might also identify whether you or your dog faces an elevated risk for certain diseases (such as kidney problems).

Such wizardry is known as DNA sequencing. It allows scientists to figure out the order of the “letters” in a DNA molecule. Those letters — called nucleotides — are the chemicals that make up DNA.

There are only four letters: adenine (A), cytosine (C), thymine (T) and guanine (G). Adenine only pairs with thymine. Cytosine only pairs with guanine. This might seem like a very limited alphabet. But the order in which those letters line up within a long string of DNA will spell out genetic instructions that tell each cell of the body which molecules it should make. And there’s plenty of space for long “words.” In humans, dogs and cats, each strand of DNA runs about 6 billion letters long.

Portions of each strand are known as sequences (as in sequences of letters). Decoding the letters in a strand is known as “sequencing” the genetic code. The letters’ precise ordering will change from one individual to the next. Still, the “words” — the instructions the sequences produce — tend to be similar between all members of a species.

Scientists can compare the order of all those letters in one person’s DNA to those in someone else’s. With billions of nucleotides, millions of those letters are going to be different, even between parents and children or brother and sister.


TINA HESMAN, SAEYBETHANY BROOKSHIRE (October 24, 2019). How DNA testing works [Blog post]. Retrieved from