Year 11 Genetics blog entry the 2nd.
A quick recap…
The students have looked at chromosomes, figured out the principles of sex determination, and been introduced to the idea that the information carried by the chromosomes is in the form of discreet instructions on how to make specific proteins.
My rationale for all this is based on my observation that learning is so much easier and so much more enjoyable and oh so much more effective when it is built upon understanding. I am, for example, currently giving private tuition to a Year 11 student who was completely stumped by a question that began, “Plants need nitrates to make protein.”
She didn’t know where to start. She thought that proteins were made at ribosomes, had never heard of nitrates, and had never been taught about the atoms required to make amino acids. Instead, she was expected to learn, by rote, with no context or understanding, that plants need mineral ions, and then, somehow, had to unpick a difficult question on the effect of water logged soil on nitrate absorption by root hair cells. Good luck with that!
There is a similar lack of scaffolding in much genetics teaching. I remember a GCSE textbook where the inheritance of eye colour was explained by parents holding cards which they gave to their children, and that some cards took preference over other cards and that you could represent the cards by using different letters….
This kind of approach gives a workable tool for solving genetic cross problems, but it leaves horrible gaps and gives rise to all manner of confusion. How can one version of a gene be “dominant” to another? How can letters determine characteristics? How does any of it link to DNA and what genes actually do?
You end up having to do an awful lot of heavy lifting to translate the simplified stuff into a form that makes any kind of sense, and very often the misconceptions are the things that stick. Even now, I will get A-level students trying to pass off the nucleus as “the brain of the cell” because it was an accessible, if meaningless, metaphor taught to them in Year 7.
So try this.
Before the lesson, I put out two pieces of paper and a pair of scissors for each student.
They will either have this…
For reasons that will become obvious, all the students on one side of the class are “females” together and the other side are all “males”.
They will also have a sheet with instructions and questions, but let’s start with the picture.
Notice that it has a karyotype at the top – either male or female – and with a gene marked towards the bottom of chromosome 7, labelled with a C on one and a c on the other.
Below this, they have two haploid versions of the same karyotype, so that the chromosome 7 either has a C or a c.
The questions on the other sheet Questions to go with Cystic Fibrosis inheritance guide 2020 start with a review…
- Look at your Karyotype – are you male or female?
Now look at the smaller karyotypes below…
- How are these different to the main karyotype?
- What kind of cells would have this kind of karyotype?
- What do we call cells that have this number of chromosomes?
- Why is this important?
A section of chromosome 7 that carries a specific bit of information has been highlighted and represented with the letter C.
- What do we call small sections of chromosomes that carry specific information?
- What do these bits of information have in common?
- What do you notice about how the letter C has been written?
They get through all of this very quickly, confidently and accurately. The previous lessons have obviously worked! Then the fun starts.
They use the scissors to cut out the two smaller karyotypes. Once they’ve done this, they have to find someone of the opposite sex. They both place their smaller karyotypes face down on the desk and select one at random from each person.
Then they turn them over and record the following information in a table
- the sex of the baby
- the combination of Cs that the baby has (i.e. CC, Cc or cc).
They retrieve their bits of paper and repeat with 3 more people of the opposite sex, so that everyone ends up with 4 babies.
This activity was greeted with great enthusiasm, hence the title of this piece, a quote from one girl leaping to her feet and announcing to the class, “So, who wants to have sex…?”
Once they’ve completed this, I ask the “mothers” to put the data on their babies on the board… Just check this out…
They can already explain the ratio of baby boys and girls – though they are very impressed with how this has somehow just happened! – but the next challenge is to explain whether the distribution of C genotypes is as expected. The brighter ones actually figure out the Punnett Square for themselves – but as understanding sinks in, the class is again amazed that the numbers match the predicted ratio so closely. Not having done it before myself, I was both delighted and relieved!
Because notice how well this works and how many different concepts it illustrates. Random fertilisation, large sample size, probability, inheritance of different alleles – and how in particular the letters they’ve been sharing are directly linked to a specific instruction on a specific chromosome. They can see it and they have done it and they therefore understand it.
But we’re not finished yet!
They already know that the letter C represents a gene, a set of instructions coding for a particular protein. Their worksheet continues…
The letter C represents a gene coding for a protein that is made in cells lining the bronchi and bronchioles. The protein transports salt from the cells into the mucus lining of the airways. This process keeps the mucus wet and slippery so that it can be easily moved out of the lungs.
- What is the function of mucus in the lungs?
- Why is it important that it can be moved?
- Why will putting salt in the mucus keep it wet?
The lowercase c represents a version of the gene that doesn’t work – i.e. the instructions for making the protein are broken.
- What effect do you think this will have on children with a cc genotype? Explain your answer.
Cystic fibrosis is a horrible disease, but it is brilliant context for so many biological topics. This part of the exercise has them reviewing gas exchange and immune system and infection and osmosis. They work out and understand the key symptoms and side effects of cystic fibrosis, even if they’ve never heard of it (many of them have, someone usually knows someone who has it). And, critically, they now have an understanding of dominant/recessive alleles at the functioning level of the gene….
As I said right at the start of this two part blog, I’ve tried this twice now on groups of varying ability, and it works really well – and by that I mean that the lesson is fun, interactive, interesting, problem-solving and effective. Admittedly, I work in a girls’ school, so you might be more tentative if working in a mixed school – I guess it would depend very much on the class. But let me know what you think!