Introduction to the myths

A fun means to teach the basics of genetics is to have students look at traits ~ above themselves. Just around every biology college student has, in one course or another, to be asked to role their tongue, look at at their earlobes, or check their fingers because that hair. Students can quickly collect data ~ above several different traits and learn about genes, dominant and recessive alleles, maybe also Hardy-Weinberg proportions. Best of all, these data don"t require microscopes, petri dishes, or stinky fly food.

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Unfortunately, what textbooks, lab manuals and also web pages say about these person traits is mainly wrong. Most of the common, visible person traits that are provided in classrooms perform NOT have a an easy one-locus, two-allele, dominant vs. Recessive technique of inheritance. Rolling your tongue is not dominant to non-rolling, unattached earlobes are not dominant to attached, right thumbs room not leading to hitchhiker"s thumb, etc.

In part cases, the properties doesn"t also fall right into the two distinctive categories defined by the myth. Because that example, students are told that they either have a hitchhiker"s thumb, i beg your pardon bends backwards in ~ a sharp angle, or a straight thumb. In fact, the angle of the thumb ranges continuously, with many thumbs somewhere in the middle. This was clearly shown in the really first paper on the genes of hitchhiker"s thumb (Glass and also Kistler 1953), however 60 year later, teachers still ask students which of the 2 kinds of ignorance they have.

In various other cases, the trait yes, really does loss into two categories, yet it isn"t figured out by genetics. Because that example, students space asked to fold their arms, climate told the the allele for having the best forearm on peak is dominant. It is true that most human being fall into two categories, ideal arm on peak or left arm on top, however the very very first study ~ above the subject (Wiener 1932) clearly demonstrated that there is little or no genetic influence top top this trait: bag of right-arm parents room just around as likely to have actually right-arm youngsters as space pairs that left-arm parents.

Some traits, such together tongue rolling, to be originally explained as installation a an easy genetic model, but later research revealed them come be much more complicated. Other traits were displayed from the an extremely beginning to not fit the an easy genetic model, however somehow textbook authors determined to ignore this. A rapid search in the standard recommendation on person genetics, virtual Mendelian Inheritance in man (OMIM), makes it clean that most of these traits do not right the simple genetic model. It is one embarrassment to the ar of biology education that textbooks and lab manuals continue to perpetuate this myths.

Who"s my daddy?

Not true.

Teachers could argue that using traits favor tongue rolling and arm folding come teach genes is a useful "lie-to-children," one oversimplification the is useful for to teach beginners, prefer teaching physics students that electrons are particles that rotate approximately the cell core of an atom in small circular orbits. They deserve to learn around polygenic traits, incomplete penetrance, and also environmental effects in later biology classes, at the exact same time they"re finding out the quantum design of electrons in your physics classes.

However, over there is one more problem through teaching inaccurate human being genetics. Let"s to speak you tell your students that arm folding is a genetic trait, with the allele for ideal forearm on height (R) being dominant to left forearm on peak (L). Results from a large number the studies show that about 11 percent of her students will certainly be R kids of two L parents; if they understand the genes lesson correctly, they will think the either they were secretly adopted, or mommy was fooling around and Dad isn"t their organic father. An ext of your students will reach this conclusion through each bogus hereditary trait that you include to the lesson. Ns don"t think this is a great way come teach genetics.

It is feasible to usage accurate hereditary traits because that a great exercise, such as blood types or DNA markers. However, some youngsters are no the biological offspring of the human being they think lock are. The many common cause of this is a woman having actually an affair v someone other than her husband or partner. Several hereditary studies, largely in European and North American populations, have discovered that the price of this "paternal discrepancy" is around 4 percent that all youngsters (Bellis et al. 2005), which means that a common classroom is likely to have at least one boy who doesn"t recognize that the human being they contact "Dad" isn"t their organic father. Much less commonly, kids are not told lock are adopted or room the product of fabricated insemination or egg donation. Therefore if you usage blood groups or DNA because that a classroom practice in genetics, few of your student may uncover out the their family belongs ~ above a lurid daytime speak show. When it have the right to be medically essential to understand who your biological parents yes, really are, a an easy classroom demonstration of introductory genetics is no the means to find out this possibly traumatic information.


Gus is L? ww Ss D? O- and likes music.

I like to usage cat coat genetics to teach straightforward genetic concepts, due to the fact that there are several quickly visible traits whose genes is well-established through cat breeders. Christensen (2000) gives an excellent description of a lesson in i beg your pardon each student in a big class records info on one cat they watch in person, climate he collects and analyzes the data. I teach a small class and have my students use come look at photos of huge numbers of cat up for adoption in different cities; that way, they can look for geographical variation in allele frequency. It is reasonably easy come score several various traits native photographs (although seeing the cat in human being is better), and also if the students don"t have accessibility to computers, you have the right to ask each college student to describe the cat they recognize best. My experience has been that even students who don"t very own a cat have actually a friend"s or neighbor"s cat that they can explain with adequate accuracy native memory.

Here space the most basic cat cloak traits for students come identify. Over there are various other traits that room rare (Siamese, polydactyly), an overwhelming to score in photographs (agouti), or more complicated (genes affecting shade patterns).

Locus GenotypesPhenotypesL (hair length) LLLlll short hairshort hairlong hairW (white) WWWwww completely white haircompletely white hairsome colored hairS (piebald spotting) SSSsss some white hairsome white hairno white hairD (dense pigment) DDDddd black, brown or orangeblack, brown or orangegray, light brown or creamO (orange) OO (♀) or O- (♂)Oo (♀)oo (♀) or o- (♂) orange or creamorange and also black, or cream and grayblack or gray

At the hair length locus, there space some proposal in the literary works that the allele for lengthy hair is more common in colder areas; this is miscellaneous students deserve to investigate using photos of cats from different cities.

The white locus is useful due to the fact that the leading W allele, i m sorry produces all-white cats, is rather rare; this helps students know that a "dominant" allele, in genetics, is one that determines the phenotype that the heterozygote, not the most usual allele in the population. The spotting, dense, and orange loci cannot be scored in cats through the W allele.

At the spotting locus, the lot of white color in cats v the S allele can selection from a couple of white toes, to white all over except because that a fancy patch on the forehead or tail. Some sources say that S/s heterozygotes have white on less than 50 percent of the body, if S/S homozygotes have actually white on an ext than 50 percent; it would be exciting for student to inspection this and see even if it is cats loss into three discrete category (no white, much less than 50 percent white, much much more than 50 percent white) or if there space a many cats with intermediate quantities of white. The pattern and also amount the white may be influenced by developmental mishaps and modifier gene (see here for much more information).

The dense pigment locus have the right to be difficult to score in bad lit photographs, but is nice clear when you view the cat in person.

The orange locus is sex-linked (it is top top the X chromosome). In the united States, cats with orange and black spot are called "calico" if they also have white indigenous the spotting locus and also "tortoiseshell" if lock don"t have actually white; in Australia, both kinds are referred to as "tortoiseshell" or "torties." male calico or tortoiseshell cats are very rare and also are generally uncovered in XXY males. Due to the fact that all 3 genotypes deserve to be differentiated in females, the is feasible to check a sample of cat for fit to Hardy-Weinberg proportions.

Summary for concerned parents

For every of these human traits, over there is a myth that one form is recessive. If that were true, 2 parents v the recessive trait might not have a boy with the leading trait. Here"s a fast listing the the reality about each the these. Can two parents who fold your arms with the left arm on top have a kid who folds with the right arm on top? Yes.

Can two parents with attached earlobes have a son with unattached? Yes.

Can 2 parents who develop red pee after castle eat beets have actually a child who renders yellow beet urine? Yes.

Can 2 parents with right pinkies have actually a boy with a bending pinkie? Yes.

Can two parents v smooth chins have a child with a cleft chin? Yes.

Can two parents without a bump inside their ear ("Darwin"s tubercle") have a kid with this bump? Yes.

Can 2 blue-eyed parents have a son with brown, environment-friendly or hazel eyes? Yes.

Can 2 red-haired parents have actually a kid with blond or brown hair? Yes.

Can 2 parents whose hair whorls counterclockwise on the ago of your head have actually a kid whose hair whorls clockwise? Yes.

Can two parents that clasp their hands v the left thumb on top have a kid who clasps hands v the appropriate thumb top top top? Yes.

Can 2 parents through hitchhiker"s thumbs have actually a child with straight thumbs? Yes.

Can two parents without mid-digital hair have a kid with hair? Yes.

Can two parents who cannot taste the bitter compound PTC have actually a son who deserve to taste it? Yes.

Can 2 parents v the large toe much longer than the 2nd toe have actually a child with the big toe much shorter than the second toe? Yes.

Can 2 parents that cannot role their tongues have a boy who can? Yes.

Can two parents without dimples have actually a boy with dimples? Probably, yet no actual research has been done.

Can two parents there is no a widow"s peak have actually a boy with a widow"s peak? Probably, however no actual research has actually been done.

Can two parents who create non-stinky urine after castle eat asparagus have a boy who provides stinky asparagus urine? Maybe not; an ext research is needed.

Can two parents v dry earwax have actually a son with wet earwax? No (or at least it"s really rare).


Bellis, M. A., K. Hughes, S. Hughes, and also J. R. Ashton. 2005. Measuring paternal discrepancy and also its public health consequences. Journal of Epidemiology and Community wellness 59: 749Ð754.

Christensen, A. C. 2000. Cats as an help to to teach genetics. Genes 155: 999-1004.Glass, B., and J. C. Kistler. 1953. Distal hyperextensibility that the thumb. Acta Genetica 4: 192-206.

Wiener, A. S. 1932. Monitorings on the manner of clasping the hands and folding the arms. American Naturalist 66: 365-370.

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return to John McDonald"s homepage This page was last revised October 29, 2012. Its deal with is It might be cited together pp. 1-5 in: McDonald, J.H. 2011. Myths of human Genetics. Sparky residence Publishing, Baltimore, Maryland.©2011 by man H. McDonald. You can probably execute what you want with this content; watch the permissions web page for details.