Phenotypic Traits: Distal Hyperextensibility Of The Thumb And Earlobe Attachment Continuous Distribution

A phenotypic, or simply trait, can be defined as a unique variant of a particular phenotypic characteristic. It is an observable, quantifiable expression of one or several genes. Common traits in humans include the fused appearance of the earlobe and the flexibility of the terminal phalanx. These thumbs are also known as “hitchhiker’s fingers” because they can be bent back quite far. It is common to believe that each of these traits can only be controlled by one locus, which is known as simple mendelian characteristics.

Distal hyperextensibility in thumb and earlobe connection can be observed as a continuous distribution. Most people have thumbs or ears of intermediate values. Glass and Kistler (1953), classified thumbs that can bend back to an angle equal or greater to 50 degrees as hitchhiker thumbs. They discovered that there were many thumbs that met the criteria and classified them as having the hitchhiker thumb trait. They noted that the thumb’s distal hyper-extensibility was not affected by age, sex, or any other type of double jointedness. After examining x-rays of one man’s thumb, they found that hyperextensibility may be due to skeletal factors. The senior author’s 3 generation family tree showed no evidence of autosomal recessive. Glass and Kistler (1953), from 192 families, determined that the thumb type is a simple mendelian feature. They used Snyder’s formulae, which was based on the HardyWeinberg principle. However, one child born to two people with straight thumbs produced a hitchhiker’s. The reason for this was incomplete penetrance. This means that non-genetic or genetic factors can also have an impact on the trait. Two types of Earlobes can broadly be classified, attached and free, even though they show continuous distribution, similar to the thumb’s distal hyper-extensibility. Walsh and Lai (1966) defined “attached” ears as those where the lowest point in the earlobe was the point at which the head attaches to it. They also examined a group of 160 families, with 347 children, from the western highlands. A second group included 6 populations from around the world. They found no differences between males, females, and ages in the distributions. They conducted a study in 160 families to test the possibility that the earlobe effect could be due a single gene. The Hardy-Weinberg principle states that the responsible genetic gene is either autosomal dominating or autosomal reclusive. They also considered the possibility of a sex linked gene causing the phenotype. Their results did not support the idea that one gene effect was responsible for the earlobe type.

There are two distinct types of tongue rolling. Positive cases allow the lateral edges to be rolled up into a tube by turning them up. Negative cases can’t be turned up. Sometimes an intermediate can be seen and it can be difficult to classify them as a distinct category. Tongue rolling, unlike the other traits, can be learned. Many children have difficulty rolling their tongues at first, but they eventually learn it. This shows that the trait isn’t just a genetic one. Sturtevant (1940), a researcher, concluded that the trait could be at least partially inherited from 62 families. 4 offspring born from non-rolling-x-rolling matrilines had the trait while 5 offspring born from rolling-x-rolling matrilines did not have the trait. He suggested that the data could also be interpreted as a result of family habits and customs or imitation. Multiple twin studies have also shown that tongue rolling does not come from a single genetic trait. Matlock (1952), showed that 73% of 33 identical (monozygotic), pairs had one of each twin that could roll the tongue. Reedy et al.

Martin (1975) and (1971), observed many identical twins with different characteristics for tongue rolling.

The current study can use all of the research results from the papers. Glass and Kistler (1953), while concluding that distal hyper-extensibility in the thumb was a simple trait of mendelian nature, their results are contradictory. The paper is not without its problems, however. The fact that the thumb has continuous distribution was not addressed by Glass and Kistler (1953), rather than classifying thumbs in “hitchhiker’s thumbs” using arbitrary values, is also a problem. Also, the accuracy of measuring the angle was an issue. The median deviation between thumb measurements from different people was 4 with some deviations over 12. Walsh and Lai (1966) also made an arbitrarily defined “attached” from “free” ears. However, their thorough analysis of the data is very useful and reliable, which makes their paper highly reliable. Their analysis was also more reliable because they used a larger sample. Sturtevant (1940) is another useful study. His small sample size allows for more detailed and precise analysis. His work is reliable.