Phenotypic Traits: Thumb Hyperextensibility and Earlobe Attachment

A phenotypic trait, or simply trait, is defined as a distinct variant of a phenotypic characteristic of an organism, meaning it is a measurable, observable expression of one or more genes. Some common human traits include the degree to which the earlobe is fused to the head, the extent to which the terminal phalanx of the thumb can be bent backwards (where thumbs that can be bent back relatively far are known as “hitchhiker’s thumbs”), and the ability to roll the lateral edges of one’s tongue into a tube. A common misconception about all three of these traits is that they are controlled by a single locus in an inheritance pattern, or so called simple mendelian traits.

In the case of distal hyper-extensibility of the thumb and earlobe attachment continuous distribution can be observed, with most individuals having thumbs or ears with intermediate values, not one of two distinct values in each case. Glass and Kistler (1953) arbitrarily classified thumbs that could bend back to an angle equal to or greater than 50 degrees “hitchhiker’s thumbs.” They found that many individuals possessed one thumb that qualified as a hitchhiker’s thumb and one that did not, but classified the individuals as having the hitchhiker’s thumb trait. They described that distal hyper-extensibility of the thumb did not appear to be affected by age or sex, and that in general people with the hitchhiker’s thumb showed no other type of double jointedness. They also observed that the trait may have a skeletal basis and may not simply be due to longer ligaments after studying x-rays of the thumbs of a man who had only one thumb classified as being hyper-extensible. A study of the senior author’s family pedigree over 3 generations indicated no evidence that the trait was not simple autosomal recessive. In a family study done by Glass and Kistler (1953) of 192 families they concluded (using the formulae derived by Snyder (1934) from the Hardy-Weinberg principle) that thumb type was a simple mendelian trait, with the allele for a straight thumb being dominant, despite the fact that one mating of two people with straight thumbs resulted in a child with a hitchhiker’s thumb. They explained that this was due to a case of incomplete penetrance, or in other words, that other genes or non-genetic factors also influence the trait.

Earlobes can be broadly classified into two types, free and attached, despite displaying continuous distribution, similarly to the distal hyper-extensibility of the thumb. Lai and Walsh (1966) classified “attached” earlobes as those where the lowest point of the earlobe was the point of attachment to the head and all others as “free.” They investigated two groups of subjects; a series of 160 families with 347 children of the western highlands of New Guinea, and a second group that consisted of 6 populations across the globe. They found no difference in the distribution of earlobe types among males and females or among different age groups. In a family study of 160 families they tested the possibility that the earlobe phenotype could be due to a single gene effect using the Hardy-Weinberg principle on the basis that the responsible gene is autosomal dominant or autosomal recessive. They also tested the possibility that the phenotype may be due to a sex-linked gene. They concluded that their results were inconsistent with any one of these single gene effects being responsible for earlobe type.

In the case of tongue rolling two fairly distinct classes can be observed. In a positive case the lateral edges of the tongue can be turned up and rolled into a tube. In negative cases the edges of the tongue cannot be turned up at all. Occasionally an intermediate is evident and classifying them into a distinct class can be difficult. Tongue rolling differs from the other two mentioned traits as it a skill that can be learned, to an extent, at least. Many children are first unable to roll their tongues but later learn to do so. This indicates that the trait is not a simple genetic characteristic. Sturtevant (1940) studied 62 families and concluded that the trait was at least partially genetic, with rolling trait being dominant, despite 4 offspring from non-rolling x non-rolling matings having the trait, and 5 offspring from rolling x rolling matings not having the trait. He mentioned that another possible interpretation of the data was that there is no truly genetic element, the correlations being dependent on family habits or customs, or on imitation of some form. Various twin studies have also demonstrated that tongue rolling is not a simple genetic characteristic. Matlock (1952) found that 7 out of 33 pairs of identical (monozygotic) twins consisted of one twin that could and one twin that could not roll their tongue. Reedy et al. (1971) and Martin (1975) also observed numerous pairs of identical twins with different phenotypic characteristics for tongue rolling.

All of the results obtained in the mentioned research papers is useful to the current study. While Glass and Kistler (1953) may have concluded that the distal hyper-extensibility of the thumb is a simple mendelian trait, their results are contradictory, making the paper somewhat problematic, but not useless. They also failed to address the fact that continuous distribution is evident in the thumb, rather classifying thumbs as “hitchhiker’s thumbs” based on arbitrary values. Furthermore there was a significant issue with accuracy when measuring the angle of the thumb, with the median deviations between measurements of the same thumbs by different people being 4, with some deviations of over 12. Lai and Walsh (1966) also arbitrarily distinguished between “attached” and “free” earlobes, but their detailed analysis of the data they collected is excellent, making their paper very reliable and useful. They also used a large sample size, making their analysis much more reliable. Sturtevant’s (1940) study is also useful, with careful and detailed analysis, despite his relatively smaller sample size. His work is reliable and seemingly accurate though, making it useful for this study, while the twin studies are immensely beneficial, as they indicate that tongue rolling is not simply a genetic characteristic, but is also environmentally influenced.