A fun way to teach the basics of genetics is to have students look at traits on themselves. Just about every biology student has, in one class or another, been asked to roll their tongue, look at their earlobes, or check their fingers for hair. Students can easily collect data on several different traits and learn about genes, dominant and recessive alleles, maybe even Hardy-Weinberg proportions. Best of all, these data don’t require microscopes, petri dishes, or stinky fly food.
Unfortunately, what textbooks, lab manuals and web pages say about these human traits is mostly wrong. Most of the common, visible human traits that are used in classrooms do NOT have a simple one-locus, two-allele, dominant vs. recessive method of inheritance. Rolling your tongue is not dominant to non-rolling, unattached earlobes are not dominant to attached, straight thumbs are not dominant to hitchhiker’s thumb, etc.
In some cases, the trait doesn’t even fall into the two distinct categories described by the myth. For example, students are told that they either have a hitchhiker’s thumb, which bends backwards at a sharp angle, or a straight thumb. In fact, the angle of the thumb ranges continuously, with most thumbs somewhere in the middle. This was clearly shown in the very first paper on the genetics of hitchhiker’s thumb (Glass and Kistler 1953), yet 60 years later, teachers still ask students which of the two kinds of thumb they have.
In other cases, the trait really does fall into two categories, but it isn’t determined by genetics. For example, students are asked to fold their arms, then told that the allele for having the right forearm on top is dominant. It is true that most people fall into two categories, right arm on top or left arm on top, but the very first study on the subject (Wiener 1932) clearly demonstrated that there is little or no genetic influence on this trait: pairs of right-arm parents are just about as likely to have right-arm children as are pairs of left-arm parents.
Some traits, such as tongue rolling, were originally described as fitting a simple genetic model, but later research revealed them to be more complicated. Other traits were shown from the very beginning to not fit the simple genetic model, but somehow textbook authors decided to ignore this. A quick search in the standard reference on human genetics, Online Mendelian Inheritance in Man (OMIM), makes it clear that most of these traits do not fit the simple genetic model. It is an embarrassment to the field of biology education that textbooks and lab manuals continue to perpetuate these myths.
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