Today in my Introductory Class we did a population genetics activity that I made up. It worked well, so I thought I’d share it.
I wanted students to understand these four causes of population variation:
- Mutation – the actual transformation of information within the sex cells, leading to new alleles
- Genetic Drift – the random change in allele % over time. I particularly emphasized the founder effect (when a small portion of the population breaks off, and they do not have the standard % of each allele, so certain traits get over-emphasized), because for early humans living in small groups, this led to a lot of diversification
- Gene Flow – the inflow of new variants, by the arrival of new members to the community
- Natural Selection – an allele that is present becomes more advantageous that other alleles and survives at a higher rate within the population, because it has become adaptive
To show the students how these play out, I decided to attempt some in-class breeding (not inbreeding). I have about 30 students in class. I gave each student a pair of alleles (2 strips of paper, held together with a paperclip). Each allele was labeled: G, g or Y. Using the Hardy Weinberg Equilibrium Principle, we started off with the following genotypes:
~25% students Homozygous Dominant (GG)
~50% students Heterozygous (Gg)
~25% students Homozygous Recessive (gg)
and one student was a mutant (YY)
GG and Gg both had a phenotype of Straight Green Hair. gg had a phenotype of Straight Purple Hair. YY had Curly Yellow Hair. To make change occur fast enough for class time, anyone with a Y allele also always had identical twin babies (linked traits among my alien population!). Students who had offspring with Y alleles brought them to the front of the room so I could give them a second set of matching alleles.
The first thing I had students do was select one of their alleles to pass on. I encouraged them to pick it randomly.
Next I said ‘everybody mate’ , and they had to go find a mate and combine their selected alleles into an offspring. Each parent was to take their unused allele and put it aside. Everyone got a kick out of picking someone to pair up with.
To make counting our results functional, I then told them to let one person keep the offspring. These students raised their hands as I called out genotypes, so we could get a count of our new generation. This was a good way to reinforce comprehension of terms as well – some rounds I said ‘who is Homozygous Dominant?’ and some rounds I just said ‘who has GG?’; it depended how confused I was making myself.
Our first round was surprisingly in line with H-W, though I think that was a fluke. So I discussed the possibility of Founder’s Effect, and how it could have impacted us. Per our mutant, We ended up with two gY babies (curly purple haired kids).
Our next round I had all the offspring holders stand up and again randomly mate. We lost one of our Y alleles at this point, so I’m glad I had they having twins, or we might not have had all the effects I was hoping for in the example. Just discussing it was a good example of how you can lose some of your variability in a small population, though. Somehow we also ended up with an odd number of offspring, so I told the leftover person that she got eaten by a tiger and didn’t get to reproduce, and was no longer a part of the gene pool. As long as I made it part of the story, students didn’t mind being taken out of our population.
We again counted up our genotypes. We had a higher percentage of Homozygous Dominant and Recessive genotypes this time, an example of Genetic Drift. And we now had 2 GY babies (Curly green haired babies). The students found that interesting – the Y gene got passed on, but mixed with a different allele that the previous generation, so the curliness stayed, but the color changed.
By the third round there were not many students participating, so it was time for some Gene Flow. I handed out the BB mutants (the fuzzy blues), who came from another population. This helped keep everyone’s attention, because more people were again participating in mating. I meant to have 7 fuzzy blue people, but I left some of them in the office by mistake, so I only had 3. It was not ideal, but it worked alright.
I again had them stand up and do random mating. This time the Y that got passed on mated with a B, so I created the twin for them, but it was only after they sat down that I told them our unfortunate news – it turns out that YB is NOT a compatible set of alleles, and none of those offspring survived. So all the YBs got knocked out of the gene pool (and that was the last of the Ys). I intentionally did not tell them this until the first YB offspring was created – because I did not want to bias the mating process. We then looked at our new allele distribution, which now included some GB genotypes with Blue-green hair (I couldn’t decide which should dominate, so neither did!).
We ran out of time, and had very few offspring due to the small population of fuzzy blues, so I didn’t actually perform the next round of mating, but we talked about the plan. The next occurrence was going to be an environmental shift: our main food source died out! But it turns out both the Y and B alleles allow individuals to process other foods in the area, a preadaptation. So anyone with a Y or a B gene survived, while those without either died. Thus, Natural Selection.
We then discussed the dilemma at hand – a YB infant does not survive, but the only aliens that can live in this environment are those with either a Y or a B gene. And then I told them about the relationship between the sickle cell mutation and malaria, to show them something similar really happens among humans (That is – people who are homozygous for sickle cell anemia often die from it, but they can’t get malaria. Those with no sickle cell alleles get malaria and can die from it. Those who are heterozygous for sickle cell anemia do not get malaria, and rarely die from sickle cell. So sickle cell alleles have both positive and negative effects in malarial regions. This is why sickle cell is so common in these regions, but virtually non-existent in other areas of the world.)
And that was my day of encouraging random mating in the classroom. I asked students afterward if it made sense, and if it helped them understand the concepts. I got a resounding yes (with a ‘plus it was fun!’ tossed in). Ultimately, we’ll see if it was effective come exam time. It’s an interesting activity because you can’t tell how it’s going to work out – we could have wiped out our Y mutants fairly early, or had some other unexpected occurrence. But I think that is part of what kept the students paying careful attention – this wasn’t something they could reproduce on their own; it was happening in real time and required all of these people, and had the potential to go horribly wrong, which made it all the more interesting. It also gave me the opportunity to discuss how nature works – when one student was sad that he didn’t get to claim a curly haired fuzzy blue baby as his own (since the YB pairs were non-viable) I responded ‘well, we don’t get to decide how nature works.’
Earlier I was thinking about how you could do this with a larger class. If you had clickers for students to use in class, I think you could do this. You could pass out the alleles as students came into class, and after each round, have those with the offspring click in their results. It would be interesting to have someone try this and see how it works.