This term, I’ve been trialling a new lunch time club – Year 11 Genetics. The idea is to attract the very bright Year 11s who are fascinated by the topic, but who quickly get frustrated with how simplistic and uninteresting the iGCSE examples are. These, after all, include the girls who, last year in Year 10, bred ebony vestigial fruit flies and worked out dihybrid cross ratios from first principle. After that, tall vs short pea plants doesn’t quite cut it!
Except that this is where I started, pushing them to challenge the idea of “gene” for height. Really? How does that work, then? Genes code for proteins. What’s a “tall” protein? How can a gene code for “Tallness”?
So we take it back to basics. Tall plants have grown more than short ones. So what is growth all about? Cell division. So how could a gene possibly be associated with that? A hormone that stimulates cell division, perhaps. Ah! Good thinking. There is a hormone, or plant equivalent, with the wonderful name of Gibberellin. It stimulates stem elongation (pause for brief discussion of “foolish seedlings”). Except that Gibberellin isn’t a protein. There is no gene for Gibberellin….
They barely blink. Well, maybe there’s an enzyme that helps make Gibberellin. Enzymes are proteins. Could there be a gene for that?
Now we’re getting somewhere. But how could there be a “recessive” version of this gene? What’s going on there? A few prompts and they get it: do “recessive” genes simply not work? Are they mutations that don’t code for a working protein? So heterozygous plants grow tall because they have a working gene which can code for a working enzyme which will produce gibberellin and stimulate lots of cell division and “tallness”.
So to understand genetics, you need to understand what DNA actually does.
They really like this because it has explanatory power. They’re not just matching up letters and traits in some trivial logic problem – they’re understanding the biochemical mechanisms. The following week, there’s more of them.
So this time I get them to review their dihybrid work from last year. And then throw linkage at them. It takes them some time, and there’s lots of brilliantly creative ideas along the way, but they get there. Perhaps these genes are on the same chromosome. So you inherit them together.
So to understand genetics, you also need to know the physical location of genes.
Week 3 and, oh look, the predicted linkage ratios don’t seem to be working out exactly. Where have that small percentage of apparently non-linked individuals come from? And why does that percentage seem to vary, depending on the trait? Crossing over and Morgan’s work on mapping chromosomes gets an airing.
So genetics is even more complicated and extraordinary than you realised!
Week 4 and we’re back to what genes code for. A 9:7 ratio of coloured sweet peas to white sweet peas. Huh? They figure out the principles of metabolic pathways and complementary epistasis.It’s just a pleasure and a privilege to watch them working together, trying out ideas, exlaining things to each other, that glorious lightbulb moment when they “get it”.
I add some points of my own. They like my analogy of a cake. You need ingredients, a mixer and an oven. If both the mixer and the oven work, you can turn the ingredients into batter, and then turn the batter into a cake. But if the oven is broken, all you get is batter. And if the mixer is broken, you can’t even get to the batter – you’re just stuck with the ingredients.
And that brings them nicely to the boil for the Nude Carp Challenge. Here it is:
Go on, have a go. You will feel ridiculously pleased with yourself when you figure it out. Will my Year 11s? I’ll find out tomorrow!