A phenotypic trait, or simply a trait, is defined as a distinct variant of a phenotypic characteristic of an organism, meaning that it is a measurable expression and observable 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 relatively backwards are known as "thumbs of the hitchhiker"), and the ability to roll the lateral edges of the tongue into a tube. A common misconception about all three of these traits is that they are controlled by a single locus in a pattern of inheritance, or so-called simple Mendelian traits. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay In the case of distal hyperextensibility of the attachment of the thumb and earlobe a continuous distribution can be observed, with most individuals having thumbs or ears with values, not one of two distinct values ​​in each case. Glass and Kistler (1953) arbitrarily classified thumbs that could bend backward at 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 they classified the individuals as having the characteristic of a hitchhiker's thumb. They described that distal hyperextensibility of the thumb did not appear to be affected by age or gender, and that in general people with hitchhiker's thumb did not exhibit other types of double jointing. They also noted that the stretch 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 hyperextensible. 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 conducted by Glass and Kistler (1953) on 192 families they concluded (using formulas 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 a mating of two people with straight thumbs produced a child with 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. The earlobes can be broadly classified into two types, free and attached, although they show continuous distribution, similarly to the distal hyperextensibility of the thumb. Lai and Walsh (1966) classified “attached” lobes as those in which the lowest point of the lobe was the point of attachment to the head and all others as “free.” They studied two groups of subjects; a series of 160 families with 347 children from the western highlands of New Guinea and a second group composed of 6 populations scattered around the world. They found no difference in the distribution of earlobe types between males and females or between different age groups. In a family study of 160 families they tested the possibility that the earlobe phenotype could be due to the effect of a single gene using the Hardy-Weinberg principle on the basis of whether the responsible gene is autosomal dominant or autosomal recessive . They also tested the possibility that the phenotype could be due to a sex-linked gene. They concluded that their findings were not.