Modelling the Inheritance of Linked and Unlinked Genes

To model the difference between linked and unlinked genes I used an activity I created that you should view here.

How it works:

By keeping some “alleles” linked and some “unlinked” we observe a difference in the inheritance pattern.

My initial instructions for this worksheet weren’t very clear but I think they are much better now. Ideally it will be printed so that the maternal genes are printed in one colour (let’s say pink, even though it’s a socially constructed convention. Let’s talk about that another time) and the paternal genes are a different colour. If I were to teach this again I would create a class set of laminated linked genes and laminated unlinked genes. The students spent far too long cutting them out themselves and I don’t think the actual process of cutting them adds anything to the learning experience.

Why is linkage important? The linkage of genes has huge implications for the inheritance of alleles. Genes which are on the same chromosome do not randomly segregated in meiosis. They physically cannot because they are on the same chromosome and so are inherited together.

Oh if only biology were so simple. NOT. That would be boooooooring! Evolution is way more sophisticated than that.

In fact, we have a process called crossing over that swaps a little bit of DNA between homologous chromosomes so your maternal and paternal DNA can be rearranged into each of your gametes. So if you can imagine, genes which are closer together are more likely to stay together. It would be very unlikely for a crossing over event to happen right at the exact spot where one gene ends and the other starts and so genes which are closer together tend to stay together.

Using an Encyclopedia analogy to describe DNA organisation

While teaching Year 12 students I noticed from their questions that many of them had not yet grasped the concept of DNA structure and organisation clearly. This was very surprising for me, mostly because (1) they were at a selective school so I naively thought that that must mean they would miraculously grasp concepts (2) we were in our 3rd term in the HSC, they should know it by now! To me, huge red flags started signalling and I approached my supervising teacher asking if she would allow me to deviate from the plan and take a lesson to revise key issues. Thankfully she agreed, although I know a little reluctantly, there is a lot of pressure to “get through” content. I couldn’t have these kids get to their final exams and not understand what a chromosome really is. After all, they were doing the core topic in biology – blueprint of life and they were also doing the genetics option. Effectively they could’ve been tested on this content twice. It was a huge gamble if I didn’t go through it.

So I prepared a series of questions aimed at pinpointing their issues. I projected these on the board and handed out laminated sheets of white paper – that I call mini-whiteboards – to each of the students and some markers and we had a quizzing and intervention session. I’d ask a question they’d respond and if it was clear that a few of them (I think I revised concepts if even 3 of them were unsure) weren’t clear we’d go through it.

From their questions it was clear that the concepts of DNA organisation was completely lost on them. They still weren’t clear on what an allele was, what a gene is and how chromosomes organise DNA or what their functions were.

It’s an abstract concept so I totally understand this confusion. If you’re teaching students about DNA, the following analogy I used might help.

All the DNA that we have in each somatic cell is a complete set called a genome. Let’s pretend this is the Encyclopaedia Britannica. You know how we have A-Z and each book has one section of the Encyclopaedia, well our DNA is organised into a similar mechanism. Each large of DNA is called a chromosome. Our chromosomes are different, just like an Encyclopaedia. The A entries are totally different form the G entries. But we need a complete set of A-Z for a complete encyclopaedia. Similarly, we need a complete set of chromosomes 1-23 for a complete genome. To add a little complexity to it, we don’t just have one set of DNA we have two. One from our mother and one from our father. So if we continue with the Encyclopaedia example this is like if we have a whole set of Britannica and another set of the World Book Encyclopaedia. We find the same entries in there, like you’ll find a definition for apple in each but they might be slightly different.

So let’s take it a step further. Each entry in the encyclopaedia is like a gene – it has a meaning and in cells that meaning is a protein. Each gene is made up of DNA in a combination of a series four bases (ATCG) these letters are what make up DNA. Similarly, definitions are made up of words.

So let’s recap.

 

Encyclopaedia Our cells
Broken up into books, A-Z Broken up into chromosomes 1-23
Come in different brands Come in different alleles. We have one of each of mum’s alleles and one of dad’s.
Each book has different entries Each chromosome has different genes
Entries are made up of words Genes are made up of nucleotides
Words are made up of letters Nucleotides are made of sugar, phosphate and bases. Each base is made of either an A, T, C or G. 

 

What do you think? My kids loved this analogy and their relief was the most satisfying aspect of my first practicum. I want to know if you think it will work for you!