The Learning Bottlenecks
-- the things that are hard to learn, and to which we should pay special attention
1. Genetics is extremely conceptual. We cannot see genes. We cannot see genotype create phenotype. We cannot see meiosis segregating different alleles, or fertilization combining different alleles. We work with models, diagrams, and simulations. Even if we work with actual organisms (e.g. ears of corn with yellow and purple kernels), we still end up deriving our understanding from the ratio of phenotypes, and not the phenotypes themselves. And, ratios are not helpful in understanding our own genetics, since few of us have the several hundred siblings required for statistical validity.
Recommendation: develop a strategy to introduce the concepts using phenotypes directly, rather than ratios of phenotypes. Do so using phenotypes that can be recognized as relevant to human phenotypes (e.g. hair color).
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2. Genotype and phenotype are just words. Without a direct connection between the two, an understanding of how one leads to the other, many students remain mystified. It is hard to relate meiosis to genetic inheritance if we don't see the relationship between chromosomes (more specifically, genes) and organismal characteristics.
Recommendation: Link the genetics unit to the transcription/translation unit, and extend this to include the effects of mutations on gene function. Use at least an example with straightforward biochemistry (e.g. pigment production, perhaps hair color). Morphological characteristics (polydactyly, earlobe attachment) require a brief excursion into embryology as well.
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3. Dominance is mysterious. This is a specific aspect of the genotype/phenotype problem; without understanding of how an allele displays dominance over another allele, one is reduced to memorizing the examples rather than the underlying principles.
Recommendation: Work with an example that is both relevant to students and has a clear explanation for why some alleles are dominant over other alleles. Pigmentation (e.g. hair color) is particularly helpful here.
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4. Different inheritance patterns as currently described in textbooks are hard to fathom. What is the difference between dominance, co-dominance, and partial dominance? What makes some things special enough that we refer to multiple alleles? Why do some texts mention that two alleles may be co-dominant at the molecular level, but at the phenotypic level show dominance or partial dominance? In actual organisms, it turns out that nearly every gene has multiple alleles, and strict dominance is not only relatively rare, but is simply one end of a continuum of various degrees of partial- and co-dominance. This continuum is referred to by geneticists as an "allelic series," for which dominance relationships depend on which alleles are being compared.
Recommendation: If we can begin with clear examples, where we can develop an understanding of why one allele might be dominant over another, we can then understand these patterns of inheritance in their own right. We can then develop a naming system if we so choose (and we may well choose to do so, since the terminology is likely to be on the High Stakes Test that very likely focuses on low-level reasoning skills like memorization and recall). At least the terminology won't drive the instruction, but rather aid in discussion of the principles that students have already learned.
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5. Punnett Squares are intelligible only to those who understand what they are trying to show. Because of their abstract nature, they are not a helpful learning tool for many students. It often becomes a matter of filling in the table by rote, rather than using the table to facilitate probability calculations.
Recommendation: Begin with a simpler, more representational form of diagram--maybe little drawings of eggs and sperm. This provides a clue that we're concerned with the genes in the gametes. Then, we can ask what possibilities there might be for the gametes...do they get the allele that originally came from Mom, or the one from Dad? It's an either/or situation, which seems to be 50:50. OK, now in the current mating, what are the possibilities for egg/sperm combinations? This type of representation might have a better chance of relating the meiotic outcome in the gametes to the genotypic outcome in the progeny. Only after we understand this concept do we see that the Punnett Square is a kind of shorthand for this more laborious reasoning process.
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6. Students often do not relate genetics to DNA. These are typically discussed in different units, at different times in the semester. It takes a special effort to connect the dots and build the larger picture. There is, it seems, some danger in beginning the genetics unit with Mendel's work (even though it is historically appropriate). Without the connection to DNA, students often infer that "genes" are some form of mysterious "trait-bearing particles" that are produced in the organs which they affect. [This conclusion is an inference drawn from the writings of college students.] Interestingly, this view is close to the "pangenesis" model of inheritance that was developed in the wake of Mendel's discoveries, and before the recognition of the role of DNA in inheritance.
Recommendation: Build the genetics unit around the molecular biology. This will automatically occur if we emphasize the mechanisms by which genotype determines phenotype.
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7. It's easy to understand that we get half our DNA from Mom and half from Dad, so it should be easy to understand that we are diploid. It should be easy to understand that gametes must be haploid. But the process of meiosis confuses everyone terribly, perhaps because students tend to get lost in the details and terminology.
Recommendation: For the unit on genetics, focus students' thinking on the function of meiosis, rather than the mechanism. The important concept is that we've got to get one copy of each gene, and not both, into the gametes. There's the allele we got from Mom, and the allele we got from Dad; each gamete gets one or the other.
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8. It is difficult to see beyond the examples to the fundamental principles. Students often memorize Mendel's peas as a wholly separate phenomenon from coat color in guinea pigs, and recognize neither as relevant to human genetics.
Recommendation: Begin with examples that appear more directly relevant to students (e.g. hair color). After the initial concepts are in hand, then bring in additional organisms, to illustrate that they, too, follow the same principles.
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9. The simplified nomenclature of an uppercase letter for one allele (e.g. A) and a lowercase letter for another allele (e.g. a) often creates the misconception that every gene has only two alleles. This seriously interferes with linking genetics to evolution (and, in fact, is the reason that Mendel's work was initially thought to contradict evolution).
Recommendation: Begin with examples for which many alleles are known to exist, and for which students already recognize that there is great diversity (e.g. hair color). Emphasize that for every gene there may be many alleles in the population overall, but that a single individual can carry only two--one from Mom and one from Dad.
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10. Alleles are mysterious. First of all, it is another odd "science word" that is unlike anything used in normal English. This, alone, poses a hurdle for some students. Second, it is unclear what alleles are if we don't know where they come from. If we begin to grasp that alleles might be the results of DNA mutation, then we may reject this notion if we have internalized the idea that a gene can have only two alleles (dominant, A, and recessive, a).
Recommendation: Emphasize the origin of alleles as mutations--errors in DNA replication. [More precisely, they mostly result from inaccurate repair of DNA damage caused by ionizing radiation, chemical mutagens, or oxidative damage by oxygen radicals.] Emphasize that alleles are "different versions" of the same gene, much as color and colour are different spellings of the same word. Since DNA damage can (and does) occur at any location within a gene (and within the whole genome), and cannot be controlled or prevented, mutations occur continuously at a low rate--generation after generation after generation. If a mutation occurs in a gamete, and makes it into a zygote that develops into an adult, then the new version of the gene (the new allele) becomes part of the population's genetic diversity. This should be relatively straightforward in the context of recommendation 9 above, examples in which a great deal of genetic variation is readily recognized by students.
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11. Mutations are mysterious. Genetics is commonly presented as the inheritance of alleles that already exist within populations. This simplifies the genetics unit in some ways, but leaves two difficulties. First, students do not necessarily see where alleles come from (#10 above), and second, the "mutations" discussed in the evolution unit seem somehow different from the stable alleles discussed in the genetics unit.
Recommendation: Follow the appearance of a new allele, from DNA damage and inaccurate repair to alteration of the protein encoded by the mutated gene to the phenotype that results from alteration of the protein. There are a number of clear opportunities for this. Hemophilia in some of the descendents of Queen Victoria is among the best, since the data point to Queen Victoria herself carrying the new mutation. Hemoglobin-S (sickle cell) is also good in that the phenotype is clearly related to the protein, though we have no record of the time of origin of the allele. It would also be appropriate to follow a hypothetical instance of a new allele arising in a gene that students have already been studying (e.g. hair color).