The tops of your fingers might appear hairless at first glance, but have you ever taken a good, hard look at them? You might be surprised to see tiny hairs sticking out of a few of them—and if they're there, there's a good chance that they came from someone closely related on your family tree. Mid-digital hair, the hair on fingers between the knuckles, tends to run in families. Taking an AncestryDNA + Traits reveals potential genetic markers linked to this phenotype.

What Is Mid-Digital Hair?

Hair that grows on the tops of your fingers near the knuckle area is called mid-digital or middle phalangeal hair. If you have it, it might be barely visible—sort of like peach fuzz—or it could be more visible because the hairs are thicker and darker. While it's a common trait, not everyone has it.

Hairy knuckles exist on a spectrum across the hand. Some people have just one or two hairs on one finger. Others have more on all their fingers except for their thumbs. Hair is most common on the ring finger, followed by the middle and pinky finger. More rarely, it's on the index finger. However, if you have hair on your fingers, one or both of your parents may have it on the same fingers.

The Genetics of Finger and Knuckle Hair

Scientists have investigated the genetics of mid-digital hair since the 1920s, when Charles Danforth first proposed that having hairy knuckles was a dominant trait. Various researchers have studied the inheritance of this trait and its prevalence in different populations.

With that said, mid-digital hair is a complex trait that doesn't follow simple Mendelian inheritance and isn't controlled by a single gene. The AncestryDNA team explored this topic by asking over 920,000 people, "Is there any hair on your fingers between the top two knuckles closest to your nail?" Analysis revealed over 2,600 DNA markers related to mid-digital hair. Based on the results, the team determined that 9% of the variation in whether people had mid-digit hair could be explained by differences in their genetics.

Other Reasons for Hair Growth on Fingers and Knuckles

Phalangeal hair (i.e., finger hair) can grow as a result of high prenatal androgen exposure. Testosterone is one such hormone in the androgen group. In both males and females, androgens stimulate hair growth.

Age also plays a factor in how much hair appears on the fingers. Androgens produced during puberty can convert vellus hair, which is fine, light-colored, and almost invisible, into terminal hair, which is thicker, darker, and more noticeable. This development makes finger hair more visible in adults than in children.

If you don’t have knuckle hair, it may be due to how your skin expresses the natural inhibitor, Dickkopf 2 (DKK2). This protein plays several roles, but is particularly prevalent in hairless skin. It blocks the pathway that is responsible for hair follicle development. If your skin has plenty of this protein on your knuckles, you aren't likely to develop hair there, even though you still have follicles present.

Interesting Facts About Having Hair on Fingers

Body hair is healthy and normal, growing across most of the human body with varying degrees of thickness. In fact, the only areas that can't grow hair are the palms of the hand, soles of the feet, lips, and eyelids. While body hair generally protects the skin and assists in temperature regulation, knuckle hair doesn't appear to offer any evolutionary advantages.

Wondering what your genes have to say about your hair? Take an AncestryDNA + Traits test to learn more about how your genetics influence the different aspects of your hair. Explore whether you have genetic markers associated with mid-digital hair and other hair-related traits, like your tendency to grow a unibrow, facial hair fullness, and how thick your hair strands are. If you've already taken a test, review your results today with an Ancestry membership.