Dear Dr. Universe: Why do people have different fingerprints? — Mary, 12, South Carolina

Dear Mary,

Did you know even identical twins have different fingerprints? It can be hard to tell twins apart, but a close look at their fingertips can reveal who’s who. The reason lies partly in their genes, but mostly from the unique way everyone’s skin grows before birth.

That’s what I learned from my friend David M. Conley, a professor at Washington State University’s Elson S. Floyd College of Medicine.

“The reason fingerprints are unique is the same reason individual humans are unique,” Conley said. “Variation is the norm, not the exception.”

There’s no single cause for your unique fingerprint design. Instead, it’s the result of both your genes and your environment. This is called multifactorial inheritance.

Look closely at the lines on your fingertips. These are called “friction ridges.” It’s hard to see, but they actually stick up above the rest of the skin.

“Fingerprints are impressions left behind when your fingers touch a glass, or when you put ink on your fingers and press them on a piece of paper,” Conley said. “Friction ridges are the actual patterns on your fingertips and palms.”

Friction ridges grow in different designs. If your parents’ fingers have a certain kind of pattern, you might be likely to have it too. That’s because genes give the basic design, and you get your genes from your parents.

Genes are like instructions written inside the body. They give directions things like eye color, nose shape, and more. (Or, if you’re a cat like me, the length of your fur or the number of toes you have.)

Genes also tell the skin how and when to grow. Before a baby is born, they grow as a fetus inside their mother’s womb. The dermis (the inside skin layer) and epidermis (the outside skin layer) grow together. Friction ridges appear where these layers meet, guided by genes.

But these layers don’t grow at the same speed for every fetus. If one layer of cells grows faster, it can stretch and pull the others. As the fetus moves, their fingers can rub against the side of the womb.

These tiny forces push the skin as it grows. Together, they mold the direction of the growing ridges. The result is a unique fingerprint unlike anyone else’s.

Everyone’s skin grows in a slightly different environment. That’s why it’s so unlikely anyone has the same fingerprints as you — about a 1 in 64 billion chance.

Koalas and chimpanzees have unique fingerprints, too. Like humans, their hands and feet are covered in friction ridges. They also spend a lot of time climbing trees, just like humans’ primate ancestors did millions of years ago. That might mean friction ridges give texture to grab rough or slippery things.

Some scientists think cats might have unique noseprints. I’m going to check that out in the mirror.

Sincerely,

Dr. Universe

“Ask Dr. Universe” is a science education project based out of Washington State University. Send questions to Dr.Universe@wsu.edu.