Your modelling career…

Last week I burbled briefly on Genetic Drift and said I would return to Hardy Weinberg in a future post. Well, here it is… (in a curious case of deja vu, part of me is convinced I’ve already written about this, but can find no evidence! if you’ve read this before, just put it down to advancing senility…).

I think most students find Hardy Weinberg difficult. It’s abstract, it’s got an equation that just seems circular and self-referencing, and it’s hard for teachers to avoid a chalk and talk approach followed by lots of practice of number crunching.

There are some super resources to help with this and I reckon you could usefully get your class to teach themselves the entire topic from this Pearson LabBench Activity.

But, as you know, I like my lessons to involve students doing things, and especially I like making them think. So I came up with this lesson plan.

First, the Powerpoint. What’s wrong with this woman?

Allele frequency intro

Of course, they can’t tell there’s anything wrong just by looking, though they come up with some very creative guesses.

The only clue is in her ethnic origin. Usually, at this point, someone gets sickle cell anaemia. Time out to talk about sickle cell anaemia, why it is a bad thing, and how it is inherited. A picture of a normal vs sickled cell, and then a heterozygote genotype.

The next slide shows a population – stress population – of individuals of varying genotypes. But we’re not going to count individuals, we’re going to count alleles.

The slide helpfully separates them to aid this. I get them to count the HbN alleles, and right at the start it’s worth pointing out the basic principle that they don’t then have to count the HbS alleles – if they know one, they know the other!

We then work out frequency. Easy, isn’t it? Again, look, if you know one, you don’t have to work out the other – they have to add up to 1.

Back to the population. What about the next generation? Are all of those individuals going to pass on their alleles? Why not? We quietly vanish the HbSHbS genotype. What’s the effect on the respective allele frequency?

So, there’s the setting. We’re looking at allele frequency in populations.

Now it’s time to start the modelling. The sheet…

Modelling Allele Frequency in Populations

introduces them to HardyWeinberg without any attempt to explain or use the equation. That can come later. For now, we’re interested in this assumption that allele frequency doesn’t change (you can demonstrate this quite nicely with a pack of cards dealt into pairs, picked up and shuffled, and re-dealt). The exercise mentions the assumptions required, and describes 3 of them. The idea of the exercise is for them to identify 4 more.

The rest is fairly self-explanatory. They divide into pairs, count out some coloured beads, and play around with them as per the instructions. You could talk a little bit about why this kind of thing can only be done as model, rather than an experiment. But they get practice in counting model alleles and working out frequency so they become very comfortable with the process.

The bit they will find difficult is imagining what events these might represent in real life. But with a bit of discussion, a bit of prompting, they figure it out. So for the first event, what could cause half your population (with their alleles!) to disappear overnight? For the second event, what can you say about the blue allele? Why aren’t those alleles being passed on? The third and fourth are more straightforward, though it helps to stress the correct terminology.

And you can then have a discussion about how realistic HW is and when/why you might find allele frequencies changing at a higher rate. This exercise …

Changes in Allele Frequency in real life

gives a few ideas.

I also wasn’t aware until recently that HardyWeinberg assumptions can be used to assess the accuracy of DNA sampling in a population. The slides here show the results of two genotypic assaying samples. You want to check whether they are valid samples. So you compare the numbers given to the Hardy Weinberg equilibrium. What does this tell you about the two samples?

HW assay

That’s enough for now. We’re being inspected and I think that’s someone at the door of the lab….


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