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They can learn about polygenic traits, incomplete penetrance, and environmental effects in later biology classes, at the same time they're learning the quantum model of electrons in their physics classes. Teachers might argue that using traits like tongue rolling and arm folding to teach genetics is a useful " lie-to-children," an oversimplification that is useful for teaching beginners, like teaching physics students that electrons are particles that rotate around the nucleus of an atom in little circular orbits.
WHAT IS RECESSIVE AND DOMINANT TRAITS MANUALS
It is an embarrassment to the field of biology education that textbooks and lab manuals continue to perpetuate these myths. 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. Other traits were shown from the very beginning to not fit the simple genetic model, but somehow textbook authors decided to ignore this. Some traits, such as tongue rolling, were originally described as fitting a simple genetic model, but later research revealed them to be more complicated. 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. For example, students are asked to fold their arms, then told that the allele for having the right forearm on top is dominant. In other cases, the trait really does fall into two categories, but it isn't determined by genetics. 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 fact, the angle of the thumb ranges continuously, with most thumbs somewhere in the middle. 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 some cases, the trait doesn't even fall into the two distinct categories described by the myth. 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. Most of the common, visible human traits that are used in classrooms do NOT have a simple one-locus, two-allele, dominant vs. Unfortunately, what textbooks, lab manuals and web pages say about these human traits is mostly wrong. Best of all, these data don't require microscopes, petri dishes, or stinky fly food. Students can easily collect data on several different traits and learn about genes, dominant and recessive alleles, maybe even Hardy-Weinberg proportions. 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. Interpretations and further applications of the conditions are given for some special migration patterns, including the stepping-stone migration mode and the generalized Island model gene flow.A fun way to teach the basics of genetics is to have students look at traits on themselves. It is further established that, with a dominant or recessive trait, fixation of alleles a and A cannot coexist as stable alternatives. The result applies whether soft or hard selection is assumed. The exact necessary and sufficient condition for protection of the recessive allele, a, is that the "weighted average" of the aa-genotype fitness values, namely ∑N i=1 ξiw i, exceed unity.
For an arbitrary migration structure between demes summarized by the backward migration matrix, M = |mij|N i,j=1 (mij = the proportion per generation of the population in deme i whose parents came from deme j), let $\underline$ = (ξ1., ξN) be the unique left eigenvector of M corresponding to eigenvalue 1, whose sum is normalized to unity. More specifically, consider allele a recessive to allele A distributed over N demes endowed with fitness parameters wi, 1, 1 for genotypes aa, Aa, and AA operating in deme i, respectively. Explicit conditions for protection of a recessive allele are determined, accessible for any migration structure. The problem of migration-selection interaction in a subdivided population affecting a recessive allele is studied.
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