Between 65 percent and 85 percent of people can roll their tongue to make a U shape. More women than men have that ability which is not an inherited one.
However, there was confusion before that it is an inherited ability. The theory can even be found in some biology textbooks. BBC attributed the inclusion of the theory in biology textbooks to Alfred Sturtevant, a leading geneticist.
He suggested in 1940 that tongue-rolling was a Mendelian trait. It could only be inherited from one parent, not both parents. At that time, people also thought that eye color, skin tone, and the presence of freckles were also Mendelian traits.
Conditioned by heredity
While Sturtevant found four people who could roll their tongues even if none of their parents could, he still concluded that tongue-rolling was partly conditioned by heredity. Another proof that it is not an inherited trait is that in a study with 33 identical twins, seven pairs did not match up. One of the twins was a roller and the other one was not. When Sturtevant learned about the study, he wrote in his book in 1965 that tongue-rolling was not a Mendelian case.
Classifying people as tongue-rollers or not is wrong because some people can curl the sides of their tongue up slightly, but they cannot make the full U. But when John McDonald, from the Department of Biological Sciences at the University of Delaware, asked 33 non-roller students to practice tongue-rolling daily, after one month, one student could do it which showed it could be a learned ability.
Other non-Mendelian traits
Other than tongue-rolling, McDonald said that hand-clasping, eye color, hitchhiker's thumb, hair color, attached earlobes are also not Mendelian traits, Mental Floss reported.
The myth about hand-clasping is that when the left thumb is placed on the right thumb on top when clasping the hand, it is determined by one gene. But the reality is there is no left-thumb-on-top gene because even identical twins have different preferences how to clasp their hands.
For blue eyes, the myth is that it is determined by a single recessive gene and a brown-eyed child cannot have two blue-eyed parents. McDonald clarified that eye color is determined by variations in several different genes and the interactions between them. It is thus impossible for two blue-eyed parents to have brown-eyed children.
Another myth is that a single gene that yields to other colors determines red hair. Also, if a mother and father both have red hair, they cannot have a child which is not red-haired. But there are a lot of variations in the gene that controls red hair pigment. The gene can be influenced strongly by genes that control brown hair. Two red-haired parents can have children with brown or blonde hair, McDonald said.
There are also speculations that a single gene determines if the earlobe of a person would be attached or unattached. But the reality is that there are more than two categories of earlobes. Instead, there is a sliding scale between attached and free earlobes. Since two previous research on attached versus free earlobes disagree which was the dominant trait, it only showed that the genetics involved are not as simple as taught.
Finally, the myth said that a thumb bent at the knuckle, known as the hitchhiker's thumb, is the result of the variation of a single gene. However, McDonald pointed out that thumb flexibility ranges differently from person to person. He said that parents with the hitchhiker's thumb can have children with straight thumbs.
For lessons in basic genetic traits in action, McDonald said that the species to study is the cat, not humans. Cats either have short or long hair, white boots or not, which are nice, simple, one-gene traits.
Who is Mendel?
While Charles Darwin believed in the inheritance of acquired variations, he did not know the process through which heredity worked. An Austrian priest, Gregor Mendel, was the one who discovered the law of genetics. The Mendelian laws he created showed how the genetic mechanisms passed the traits on to population.
The theory of Darwin became even more scientific with Mendelian genetics, but it jettisoned Darwin's own belief in the inheritance of acquired characteristics.
Meanwhile, Mendelian traits are determined by a single gene and inherited in a single fashion. It follows the law of inheritance that Mendel described. If both the mother and the father are heterozygous, three-fourths of their children will have the dominant version of that trait, while the recessive version will go to the one-fourth. Parents who have two identical copies of the gene are also homozygous.
But if one parent is homozygous for the dominant version of the gene, while the other parent is homozygous for the recessive form, all of their children will be heterozygous.
[researchpaper 리서치페이퍼=Vittorio Hernandez 기자]