John H. McDonald
Karla Boyd
Department of Biological Sciences
University of Delaware
Some people can roll their tongue into a tube, and some people can't. The proportion of people who can roll their tongue ranges from 65 to 81 percent, with a slightly higher proportion of tongue-rollers in females than in males (Sturtevant 1940, Urbanowski and Wilson 1947, Liu and Hsu 1949, Komai 1951, Lee 1955).
Tongue rolling is one of the human traits most commonly used to demonstrate simple Mendelian genetics. It is said to be a simple two-allele character, with the allele for rolling (usually given the symbol T or R) being dominant over the allele for non-rolling (t or r). Here are a few examples of the many web pages that perpetuate this myth:
Most people, when first asked, either can easily roll their tongue (here called "R"), or cannot roll it at all ("NR"). However, some people, especially children, cannot roll their tongue when first asked but later learn to do so (Sturtevant 1940). Komai (1951) found that the proportion of tongue-rollers among Japanese schoolchildren increased from 54 percent at ages 6-7 to 76 percent at age 12, suggesting that over 20 percent of the population learns to tongue-roll during that age range. There are also some people who can only slightly roll the edges of their tongue and cannot easily be classified as rollers or non-rollers (Reedy et al. 1971).
Sturtevant (1940) compared parents and offspring, with the following results:
| Parents | R offspring | NR offspring |
| R x R | 28 | 5 |
| R x NR | 33 | 22 |
| NR x NR | 4 | 9 |
He concluded that tongue rolling was at least partially genetic, with rolling dominant to non-rolling, despite the four R offspring of NR x NR parents.
Komai (1951) performed a similar study with much larger sample sizes, and found similar results:
| Parents | R offspring | NR offspring |
| R x R | 928 | 104 |
| R x NR | 468 | 217 |
| NR x NR | 48 | 92 |
In both family studies, individuals with tongue-rolling parents are much more likely to be tongue-rollers than individuals with non-rolling parents. It is difficult to imagine how the common family environment could influence tongue-rolling, so this resemblance between relatives suggests that there is a large genetic influence on tongue-rolling. However, the large number of tongue-rolling individuals with two non-rolling parents is inconsisent with the myth that this is a simple one-gene, two-allele genetic character, with rolling completely dominant to non-rolling. The discrepancy could be due to more complicated genetics, involving multiple alleles or multiple genes, or some kind of environmental influence.
Matlock (1952) found that out of 33 pairs of monozygotic (identical) twins, 7 pairs consisted of one R and one NR twin. This clearly establishes that there are important non-genetic influences on tongue rolling, and it convinced Sturtevant (1965) that his initial interpretation was incorrect. Reedy et al. (1971) and Martin (1975) also found numerous pairs of monozygotic twins who differed in tongue rolling. Dizygotic twins were twice as likely to differ in tongue-rolling ability as monozygotic twins (Reedy et al. 1971), which is additional evidence that there is some genetic influence on this trait.
| Matlock (1952) | Reedy et al. (1971) | Martin (1975) | Reedy et al. (1971) DZ twins | |
| Both twins R | 18 | 43 | 15 | 81 |
| Both twins NR | 8 | 11 | 5 | 21 |
| One R, one NR twin | 7 | 7 | 8 | 30 |
Family studies clearly demonstrate that tongue rolling is not a simple genetic character, and twin studies demonstrate that it is influenced by both genetics and the environment. Despite this, tongue rolling is probably the most commonly used classroom example of a simple genetic trait in humans. Sturtevant (1965) said he was "embarrassed to see it listed in some current works as an established Mendelian case." Tongue rolling is unsuitable for classroom use as an illustration of simple Mendelian genetics.
Komai, T. 1951. Notes on lingual gymnastics. Frequency of tongue rollers and pedigrees of tied tongues in Japan. J. Hered. 42: 293-297.
Lee, J. W. 1955. Tongue-folding and tongue-rolling in an American Negro population sample. J. Hered. 46: 289-291.
Liu, T. T., T. C. Hsu. 1949. Tongue-folding and tongue-rolling in a sample of the Chinese population. J. Hered. 40: 19-21.
Martin, N. G. 1975. No evidence for a genetic basis of tongue rolling or hand clasping. J. Hered. 66: 179-180.
Matlock, P. 1952. Identical twins discordant in tongue-rolling. J. Hered. 43: 24.
Sturtevant, A. H. 1940. A new inherited character in man. Proc. Nat. Acad. Sci. 26: 100-102.
Sturtevant, A. H. 1965. A History of Genetics. Harper and Row, New York, NY.
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This page was last revised January 30, 2007. Its URL is http://udel.edu/~mcdonald/mythtongueroll.html