How to teach about dominance

Yesterday the team of people teaching BIOL 121 had our second meeting to discuss our new learning objectives. One issue that hadn't been included in the list of objectives is dominance. I added it to the list on our Instructors' Blog, stated as follows:

Students should be able to define dominance as a particular relationship between the effects of two alleles; dominance is said to exist when the phenotype of the heterozygote is the same as that of a homozygote for one of the alleles (the 'dominant' one). They should also be able to explain that dominance usually results when a single copy of the normal allele is sufficient to give the normal phenotype when combined with a defective allele, and to predict phenotypes when given such information.

Students find this very difficult, I think mainly because they are encouraged in high school to blindly accept "Mendel's Rules", and think of dominance as resulting from some mysterious gene inactivation process. At the meeting I put forward the way I have been trying to teach this concept. I was (slightly) mortified to discover how many assumptions my explanation relied on (assumptions fortunately not shared by my colleagues), so I've been trying to build a better explanatory framework, using ideas they raised.

Here are the figures I would use. The first three figures would be introduced at the end of the first class about how genotypes determine phenotypes (yes, I know that's an oversimplification that ignores the massive effect of environment in real organisms...).

Figure 1: Introduce lactase and lactose intolerance:

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Figure 2: Show a graph of how lactose digestion depends on the amount of lactase:

Students may need to be told that each tube contains the same amount of lactose.

(I don't know if students should be told that this is fake data.)
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Figure 3: Show the questions I'll ask about this information at the start of the next class.
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Here are the figures I would show at the start of the next class:

Figure 4: Show the first question again.

Students could be asked this question first, before being given the guidance suggested on the next figures.
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Figure 5: Remind them of the graphed data.

I've made the original graph pale, and superimposed on it the labels and numbers appropriate to thinking about the amounts of lactase in adults.
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Figure 6: Add bars showing how much lactose would be digested by each amount of lactase.

Students could be guided by asking them how high these bars should be.
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Figure 7:

Now we're back to the original question. After the guidance all students should be able to see that adults with 5µg/ml lactase digest lactose almost as well as adults with 10µg/ml lactase.
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Figure 8:

This question is intended to connect their understanding of genotypes to phenotypes. The answers should be: 0 = -/-, 5 = +/-, 10 = +/+.
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Figure 9:

The +/- heterozygotes have almost as much ability to digest lactose as the +/+ homozygotes. So they will be lactose-tolerant, and we will describe the + allele as being dominant to the - allele.

This is a lot of figures. Ideally the students would have the time to try to figure most of the steps out themselves; the figures are my ideas of the steps their thinking should take.

This lesson should also build the idea that dominance and recessiveness are not properties of alleles in isolation, but properties of relationships between pairs of alleles.

I usually tell students to ignore confusing terms like 'partial dominance' and 'incomplete dominance' and 'co-dominance', and to instead just describe any other interactions between alleles and phenotypes using more informative descriptions such as 'blending' (e.g. many pigments) or 'both phenotypes are present' (e.g. blood types A and B).

The numbers on the graphs are made-up data. Real human lactase assays are usually normalized to the ratio of sucrase to lactase, which is too confusing to present here. But here's a nice graph showing how lactase levels decline and sucrase levels rise in rats (from a page by R. Bowen at Colorado State).