John H. McDonald
Charlene R. Kemmerle
Department of Biological Sciences
University of Delaware
Some people have an index finger that is longer than their ring finger; we'll call this long index finger, or L. Others have an index finger that is shorter than their ring finger (S). This is said to be a sex-influenced trait. In males, the allele for S is said to be dominant, while the allele for L is said to be dominant in females. Thus SS and SL males will have short index fingers and only LL males will have long index fingers, while SS females have short index fingers and SL and LL females have long index fingers.
Here are a few examples of web pages that perpetuate this myth:
It is difficult to measure finger length accurately. Placing the hand palm-down and seeing which finger extends further is a problem, because a slight shift in the angle of the fingers dramatically changes the relative position of the fingertips. Measuring from the knuckle to the fingertip, or from the crease between finger and palm to the fingertip, is imprecise because skin moves around. The most precise measurements come from X-rays, but this is obviously impractical for classroom use. Each technique measures from a different place on the finger and therefore would give different results on the same hands.
The person on the left has an index finger longer than the ring finger (L), while the person on the right has a shorter index finger (S).
The myth divides hands into two categories, those with index fingers longer than ring fingers, and those with index fingers shorter. In reality, there is a continuous distribution of relative finger lengths:
Histogram of index:ring ratios; data extracted from Fig. 1 of Paul et al. (2006). All ratios are less than 1, presumably because finger length was measured using X-rays. Note that there is not a bimodal distribution of long and short.
Phelps (1952) first proposed the sex-influenced mode of inheritance. Under this hypothesis, the male offspring of matings of S males with S females should all be S, and 60 out of 60 males were. However, the female offspring of a mating of L males and L females should all be L, but 2 out of 5 female offspring were actually S (Phelps 1952, table 6). The products of other matings fit the sex-influenced single-gene model fairly well.
Paul et al. (2006) estimated heritability of the index/ring finger ratio in female twins. As with other recent studies, they treated the ratio as a continuous variable, rather than dividing individuals into two or three categories. The ratios were significantly correlated between pairs of twins, and the correlation was stronger for monozygotic twins than for dizygotic twins, which indicates that there is some genetic basis for the variation in this ratio. Heritability was estimated to be about 66 percent.
There are many recent studies that have looked at the relationship between the index/ring finger ratio and sex-related traits, such as homosexuality, number of sexual partners, testosterone and estrogen levels, sperm count, ADHD and autism (see the Wikipedia entry on digit ratio. This modern research consistently treats digit ratio as a continuous character, influenced by a complicated mix of genetic and environmental factors. Digit ratio is not the simple one-gene, two-allele, sex-influenced trait described in the myth, and it is unsuitable for classroom use as an illustration of simple Mendelian genetics.
Kloepfer, H.W. 1946. An investigation of 171 linkages in man. Annals of Eugenics 13: 35-71.
Paul, S. N., B. S. Kato, L. F. Cherkas, T. Andrew, and T. D. Spector. 2006. Heritability of the second to fourth digit ratio (2d:4d): A twin study. Twin research and human genetics 9: 215-219.
Phelps, V. R. 1952. Relative index finger length as a sex-influenced trait in man. American Journal of Human Genetics 4: 72-89.
Return to the main Myths of Human Genetics page
Return to John McDonald's home page
This page was last revised January 30, 2007. Its URL is http://udel.edu/~mcdonald/mythfingerlength.html