At some point, you probably learned about the idea of dominant and recessive genetic characteristics. A common example is tongue-rolling — those who can do it are said to have a dominant genetic trait. Except that's all wrong. And so are a lot of other things your teacher called dominant traits, too.
University of Delaware geneticist John McDonald has a helpful website where he chronicles common myths about genetics. One of his favorite things to debunk is the notion that there are simple dominant and recessive traits such as the rolled tongue, bent pinkies, "hitchhiker's thumb" and even eye color.
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.
About the tongue-rolling myth, McDonald writes:
Most people, when first asked, either can easily roll their tongue (here called "R"), or cannot roll it at all ("NR"). The proportion of people who can roll their tongue ranges from 65 to 81 percent, with a slightly higher proportion of tongue-rollers in females than in males (Sturtevant 1940, Urbanowski and Wilson 1947, Liu and Hsu 1949, Komai 1951, Lee 1955). However, some people, especially children, cannot roll their tongue when first asked but later learn to do so (Sturtevant 1940). Komai (1951) found that the proportion of tongue-rollers among Japanese schoolchildren increased from 54 percent at ages 6-7 to 76 percent at age 12, suggesting that over 20 percent of the population learns to tongue-roll during that age range. That some people learn to roll their tongues after first being unable to is the first evidence that this is not a simple genetic character. There are also some people who can only slightly roll the edges of their tongue and cannot easily be classified as rollers or non-rollers (Reedy et al. 1971).
Want to find out more about genetic myths? Check out McDonald's Myths of Human Genetics website.