Hitchhiker's Thumb
What Do Classrooms Say?
Classrooms teach that thumbs can be divided into two clear categories, hitchhiker's and not hitchhiker's. Hitchhiker's thumbs are those that can bend backwards with a large angle between the two phalanges. Straight thumbs are those that cannot bend at all. According to textbooks, hitchhiker's thumbs (H) are the dominant trait while straight thumbs are the recessive trait (A). |
What Does the Research Say?
Parents S offspring H offspring Percent S
SxS 281 32 90%
SxH 71 37 66%
HxH 1 30 3%
2. Beckman (1960) also conducted a family study on the hitchhiker's thumb. His study used data from one hundred families that each had one child of about the age of three. His data are in the following chart:
_____________________________________
Parents S offspring H offspring Percent S
SxS 50 8 86%
SxH 18 17 51%
HxH 3 4 43%
Conclusions:
Thumbs cannot be divided into just two categories: "hitchhiker" and "non-hitchhiker". From the picture below it's clear that there is a whole spectrum of thumbs between the two categories. While some thumbs cannot bend at all, others can bend well past 90 degrees. While there is probably some genetic influence on the hyperextensiblity of the thumb, the family studies above show that it does not fit the simple one-locus, two-allele myth.
- Glass and Kistler (1953) conducted a family study on 450 families. They decided that anyone with one or both thumbs having an angle equal to or greater than 50 degrees had the hitchhiker's thumb trait. From their study they collected the following data:
Parents S offspring H offspring Percent S
SxS 281 32 90%
SxH 71 37 66%
HxH 1 30 3%
2. Beckman (1960) also conducted a family study on the hitchhiker's thumb. His study used data from one hundred families that each had one child of about the age of three. His data are in the following chart:
_____________________________________
Parents S offspring H offspring Percent S
SxS 50 8 86%
SxH 18 17 51%
HxH 3 4 43%
Conclusions:
Thumbs cannot be divided into just two categories: "hitchhiker" and "non-hitchhiker". From the picture below it's clear that there is a whole spectrum of thumbs between the two categories. While some thumbs cannot bend at all, others can bend well past 90 degrees. While there is probably some genetic influence on the hyperextensiblity of the thumb, the family studies above show that it does not fit the simple one-locus, two-allele myth.
Sources
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 4: 192-206.
Jones, Kristin. "Genetics Unit." Biology Class. Novi High School, Novi. 8 Dec. 2010. Lecture.
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 4: 192-206.
Jones, Kristin. "Genetics Unit." Biology Class. Novi High School, Novi. 8 Dec. 2010. Lecture.