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Genetics of hitchhiker's thumb
Summary
You may have been told that thumbs come in two shapes, "hitchhiker's thumbs," which bend back at a sharp angle between the two segments, and straight thumbs. This is not true! Multiple studies have shown that thumbs vary smoothly from straight to a sharp angle, with plenty of thumbs at every angle in between. You may also have been told that straight thumbs are genetically dominant, but family studies have shown that that's also not true! Don't worry if your family includes hitchhiker's thumb parents with a straight thumbed child, and if you're a teacher, don't use hitchhiker's thumb as a genetic example .
Are there just two kinds of thumbs?
Stick your thumb up, like you're giving science the thumbs-up sign, and look at the angle between the first and second finger bones (phalanges). If your second thumb bone bends back at a sharp angle, that's called a "hitchhiker's thumb." Some biology teachers divide people into two categories, those with straight thumbs and those with hitchhiker's thumbs.
Harris and Joseph (1949) X-rayed the thumbs of 294 people and measured the angle between the first and second phalanges. They found a continuous distribution, with most individuals having intermediate values:
Glass and Kistler (1953) did a similar study, except they used a protractor held against the outside of the thumb to measure the thumb angle, and obtained the following results:
Instead of looking at their results and reaching the obvious conclusion that you can't divide thumbs into two distinct categories, Glass and Kistler (1953) arbitrarily called all thumbs with an angle equal to or greater than 50 degrees "hitchhiker's thumbs." They gave no explanation for why they picked 50 degrees as the dividing point. They noted that many individuals would then have one hitchhiker's thumb and one straight thumb; they classified these individuals as having the hitchhiker's thumb trait. Glass and Kistler (1953) had different people measure the thumbs, and the repeated measurements often differed by several degrees, which would mean that many people would be considered to have a hitchhiker's thumb by one observer but not by a different observer.
I searched the internet for pictures of thumbs (it was easy, because lots of people give the thumbs-up sign when they get their picture taken) and arranged them from straightest to most bent. As you can see, there's a range of thumb angles, from straight to nearly 90 degrees, with no clear division between hitchhiker and non-hitchhiker thumbs. Asking someone "Do you have hitchhiker's thumb or straight thumb" is like asking someone "Are you tall or short"; some people are definitely tall or definitely short by anyone's standards, but a lot of people would say "I'm somewhere in between, what's your arbitrary dividing line between tall and short?"
Family studies
Glass and Kistler (1953), after arbitrarily deciding that anyone with one or both thumbs having an angle equal to or greater than 50 degrees had the hitchhiker's thumb trait, collected the following family data:
| Parents | S offspring | H offspring | Percent S |
|---|---|---|---|
| S×S | 281 | 32 | 90% |
| S×H | 71 | 37 | 66% |
| H×H | 1 | 30 | 3% |
They concluded that thumb type was a simple Mendelian trait, with the allele for S dominant. The single individual who didn't fit this model (the S offspring of two H parents) was explained as an example of incomplete penetrance, meaning that other genes or non-genetic factors also influence the trait. Glass and Kistler (1953) were too polite to mention incorrect paternity as another possible explanation.
The only other family study on hitchhiker's thumb that I am aware of is Beckman et al. (1960):
| Parents | S offspring | H offspring | Percent S |
|---|---|---|---|
| S×S | 50 | 8 | 86% |
| S×H | 18 | 17 | 51% |
| H×H | 3 | 4 | 43% |
If hitchhiker's thumb was recessive, two parents with hitchhiker's thumb could not have a child with a straight thumb. The three S offspring of H×H parents are inconsistent with a two-allele model in which the allele for S is dominant.
Further research
Two careful studies with large sample sizes have shown that thumb angle is a continuous trait, so I don't think anyone needs to repeat that. It's a trait that is easy to measure and shows a lot of variation, so it might be kind of fun to include it in a large genome-wide association study of a bunch of characters. Other than that, I can't think of any research to do on thumb angles.
References
Beckman, L., J.A. Böök, and E. Lander. 1960. An evaluation of some anthropological traits used in paternity tests. Hereditas 46: 543-569.
Glass, B., and J.C. Kistler. 1953. Distal hyperextensibility of the thumb. Acta Genetica et Statistica Medica 4: 192-206.
Harris, H., and J. Joseph. 1949. Variation in extension of the metacarpo-phalangeal and interphalangeal joints of the thumb. Journal of Bone and Joint Surgery 31: 547-559.
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This page was last revised October 26, 2026. ©2026 by John H. McDonald. Content is available under the Creative Commons Attribution-Noncommercial license.