Using formative assessment for individualized instruction

As part of the unit on Natural Selection and Evolution, I use antibiotic-resistant bacteria as an example of evolution that we have been able to see and measure in real time. At this point in the year, we haven’t talked much about bacteria, so my students don’t have a lot of background knowledge about them. The past couple of years, my bio colleagues and I have used the Antibiotic Sensitivity lab to give students a hands-on experience of working with bacteria, agar plates, and antibiotic disks. (If you haven’t done this lab before, both Flinn Scientific and Carolina have kits that give you everything you need.)

Students read the results of their lab about 24 hours after they plate the bacteria. They have to look for and measure any zone of inhibition around the antibiotic disks that they placed on the agar. From this, they have to conclude whether the antibiotic is effective against the bacteria.

Students measure the diameter of the zone of inhibition

I did not have students prepare a formal lab report for this assignment. Instead, there were a few targeted questions asking about how they know if the bacteria is resistant to an antibiotic, and how they would determine which antibiotic is most effective against the bacteria. I was looking for evidence that students understood that if an antibiotic is effective against bacteria, there will be a clear zone of inhibition around the disk where the bacteria were killed. If the bacteria was resistant to the antibiotic, then there would be no zone of inhibition around the disk.

Within the first few assignments I started grading, I quickly saw that students were reversing the two results. It was one of those “ooooh dear” moments, so I started spot-checking answers across all of my classes to see if it was a fluke (I was crossing my fingers!), or if there was a real pattern emerging. Sure enough, a significant number of students thought that if there was a zone of inhibition, that meant the bacteria were “resisting” that antibiotic.

Ooooooops. Double oops, when I considered the fact that there were questions on the upcoming test that assessed this concept. But I was also running out of time – there was only one class period before the test. At first I thought that I would reteach the concept to the whole class, although that would take away time that I had planned to give them for review. Thankfully, the lightbulb went on and I realized I could do a quick spot check to figure out which students did not understand. Then I could pull them aside for some one-on-one review.

I teach at a 1:1 school, so I knew I could use a Google form for quick answers. I started with a photo of a bacterial plate with five antibiotic disks on it, showing a range of zones of inhibition. I edited the photo to number each disk. I wrote four brief questions: (1) which antibiotic is the bacteria resistant to (they could choose as many as they wanted); (2) if the bacteria are resistant to an antibiotic, what will you see on the petri dish (short answer); (3) which antibiotic is the most effective against the bacteria (multiple choice – they could only choose one); and (4) explain how you know which antibiotic is most effective against the bacteria (short answer).

I had students fill out the Google form at the very beginning of class. After everybody had submitted their responses, I could quickly see who understood and who did not.

I also exported the data to a Google sheet so I could identify which students answered incorrectly. (I set the form to automatically collect their email address.) For those students, I had a paper copy of the image I used. I went to each student individually and went over the answers to make sure they had the correct information. They got an annotated copy of the correct answers to put in their notebooks so they can refer to it when they are studying for their test.

The best things about this were that I could find out exactly who needed help, and that it only took a few minutes of class time to find out the information!

Smelly Balloons – introducing cell membranes and permeability

Oh. My. Goodness. This is one of my favorite activities for introducing cell membranes and diffusion! The original idea is from Flinn Scientific, but the “lab” provided only the barest outlines of what you could do with this activity. This activity is FUN but definitely needed a little supplementing to make it more educational. I used the activity as a standalone for several years, but a couple of years ago I beefed up the analysis with a worksheet.

So first, let me tell you the fun part. You take latex balloons and fill each one with a little bit of flavoring extracts. Students try to identify what each smell is – it’s always entertaining to watch them sniff the balloon and argue with each other what it smells like.

I have a stockpile of four or five different flavor extracts so I can change them up. Try to use ones that smell distinct from each other – lemon and lime smell very similar. Word of caution, though – I used maple extract one time. Do not recommend . . . unless you want your classroom to smell like pancakes for a few days. Whew, that smell LINGERS! I use four balloons per class and tape them at different places around the room.

You can reuse balloons if you have back-to-back classes, because the smell will still be strong enough. However, I like to make fresh balloons in front of the students so they can see that I’m putting the extract inside of the balloon. It leads to a great discussion – “You saw me put the liquid INSIDE, so explain how you can smell it outside of the balloon!” – that they would miss out on if you just gave them a pre-prepared balloon. And occasionally we get the unintentional comedy when I let go of the blown-up balloon before I’ve tied it in a knot.

I added a second procedure that rounds out the idea of semi-permeability of cell membranes. I add two or three stations with a scale and some water balloons. I’ve recorded the mass of the water balloons ahead of time, and students have to weigh the water balloon and record the initial and final mass. (OK, full disclosure here: sometimes I have to fudge this a little bit. I tape a weigh boat to the scale, because if the balloon is sitting at a different location on the weigh plate, the mass will be different. And to be honest, if I forgot to do this ahead of time, I’ll weigh the balloon right before class and pretend I did it much earlier.)

After students have complete both of those activities, we have a discussion about why the extract molecules “escaped” but the water molecules didn’t. If they’re having trouble with an explanation, I project a photo of latex under an electron microscope so they can see that what seems like a solid sheet of material actually has spaces between the molecules. That leads into a discussion of the relative sizes of the extract molecules and water molecules.

The final step is for students to complete an analogy map. They have to explicitly compare the balloon to a cell membrane, the flavor extract to small molecules, and the water to large molecules.

This activity is a great way to introduce students to cell membranes and permeability, as a lead-in to discussing osmosis and diffusion. It doesn’t take much time to set up, and it’s enough of a discrepant event to get students thinking. Plus, it’s very memorable to students – you can refer back to it later when they have to think through diffusion, facilitated diffusion and osmosis, and even active transport.

Before-During-After Drawings – Helping students differentiate between diffusion, osmosis, and active transport

Students seem to have difficulty sorting through the different types of cell transport, because it’s such an abstract concept. Even after reading the textbook, taking notes, and doing different activities, my students didn’t understand the difference between diffusion, osmosis, and active transport. I created a handout for my students to use as part of our review for the test that had them visually explain what was happening during each process.

The basic format is one of the formative assessments Paige Keeley sets out in her book, Science Formative Assessment (volume 2). She calls them “B-D-A Drawings”. Rather than do each process separately, I created one document to compare the three processes. It was a spur-of-the-moment creation, so I hand-drew the drawings for the “before” panels. One benefit of giving students the “before” drawings is that they’re all using the same basic shapes for solute and water, and the same number of molecules, and I can set it up to guide them toward what their “during” and “after” drawings will contain.

Students got this basic template, with the “before” drawings

I did a jigsaw activity for this handout – I counted off students as 1, 2, or 3, then put each number at a separate table. Each table was assigned one drawing to complete. As each table worked on their drawings, I circulated through the room to answer questions they had, or to ask groups questions to prompt them to think about what would happen for their assigned process. When the group working on diffusion seemed stuck, I did a quick demo with a beaker of water and some food coloring. The osmosis group had the right idea about water moving (instead of solutes), but when I saw that their drawings did not change the water level, I asked them what would happen to the water level on each side.

Students used their science notebooks to help them think through their processes. It took them about 5-10 minutes to discuss what they thought would happen and draw the “during” and “after” diagrams. It also prompted a good discussion about equilibrium – how it would be different for each process, and how the active transport process wouldn’t reach equilibrium.

After each group had completed their set of diagrams, I regrouped students so there was one person with each diagram at a table group. Each student had to explain their diagrams to their tablemates and answer any questions. After each student had explained their process, students had to complete the diagrams for the two processes they didn’t have.

My answer key – oops, forgot to take pictures of student samples!

Overall, this activity gave students a visual explanation of the differences between diffusion, osmosis, and active transport. With larger classes, you could create multiple groups for each process to keep group size small enough to keep students focused on the work.

Red light, yellow light, green light! Using a Learning Objectives “Traffic Light” to have students self-evaluate knowledge

As we get closer to the end of a unit, with the test date looming in sight, students start wondering “What’s going to be on the test?” Of course, they start off by asking (demanding?) if I’m going to give them a study guide. Side note: I occasionally probe what they mean by “study guide” by asking them what kind of study guides they received in middle school. It varied – some got a problem sheet, some got a list of questions that ended up being the same questions they saw on the test. Very few students reported being taught how to make their own study guides.

One of the strategies I learned when I participated in the Academy for Excellence in Biology Teaching (through the Stanford Center to Support Excellence in Teaching) was the “Traffic Light”. The Traffic Light is a list of learning objectives in a grid format. This is the third year of using Traffic Lights as study guides for our classes – although the details have morphed a little bit over time, the basic format remains the same.

I use Excel to create the traffic light because it’s easy to make the grid and reformat as needed. The goal is to fit the traffic light on one page. Each learning objective gets its own line in the sheet, in the center column of the grid. Over time, our Traffic Light handouts have gotten bigger, mostly because we’ve gotten more detailed in the information our learning objective contains. We want students to know exactly what they need to learn. The right column is blank so students can write in their own information. The left column is blank, and that’s where the “traffic light” tag comes into play. I instruct students to read the learning objective and evaluate how well they know or understand the content of the learning objective. They fill in the left column with either red, yellow, or green: “Red light” means they don’t understand that concept at all; “Yellow light” means they know something about it, but don’t feel especially confident that they understand it; and “Green light” means they are 100% sure they understand that concept.

At first, I found that students wanted to impress (or maybe distract) me, so they would give themselves lots of green lights. Now I remind them that the traffic light is just for them, and that I might not ever see it, so it’s important for them to be honest with themselves about how well they understand. If I’m working one-on-one with a student during office hours, I can ask them questions about a particular learning objective to draw out how well they understand the content, and help them determine where they really fall on the learning spectrum.

Once a student has completed the traffic light, I point out that they’ve just prioritized what they need to study for the test. Spend the most time on the red light material, fill in the missing parts on the yellow light material, but don’t forget to review the green light material as well.

Not exactly red/yellow/green, but this student has evaluated what they know

Students use the right column in a few different ways. Some of them will go through their notebook and write which pages or activities relate to each learning goal. That way they know which notebook pages to study. I also remind them to look at the labs we’ve done, since those won’t be in their notebooks. Some students jot down notes or summaries in the right column, using it for retrieval practice of what they know. After they finish, they can go back through their notebooks to fill in the blanks or correct misconceptions. A lot of these students will also use the traffic light as a jumping off point to make a more detailed study guide for themselves.

Using the Traffic Light to summarize what they know

Up to now, we’ve created the Traffic Light as a retrospective view – once we’ve created the unit test, we make the Traffic Light to focus students specifically on what they need to know for the test. For example, if I taught how temperature, pH, and substrate concentration affect the rate of enzyme reaction, but the test question only assesses temperature, then the learning objective will focus students on just temperature. On the one hand, students feel like they aren’t “wasting their time” (minor eyeroll here) studying information that won’t be on the test. On the other hand, I don’t want to create the impression that some of the concepts taught weren’t “important enough” to be on the test. I’m sure the truth falls somewhere in between those two extremes, but sometimes it feels like we’re aiming for fewer student complaints. Ideally, I would like to use the Traffic Light as a prospective view by introducing it at the beginning of a unit so students can track their progress as they learn. It would be more of a roadmap for the unit so students can see where we’re going from the beginning of the unit.

Teaching students how to identify parts of an experiment

One thing I’ve noticed over the past few years is that my students (9th graders) have a hard time figuring out the different components of an experiment. This leads to difficulty in writing a hypothesis, knowing how to graph their data, and ultimately, in writing a scientific conclusion in their lab reports.

When I was looking for review materials at the end of our first unit, I found a worksheet to help with this issue. The worksheet gave a description of an experiment, and asked students to identify the independent variables, dependent variables and constants, describe the control and experimental groups, and write a proposed hypothesis. (Of course, I looked at so many websites to find this that I now can’t remember the actual source of this worksheet.) I had my students do the review worksheet, and it seemed to help them.

After the activity, my colleague (we both teach multiple sections of on-level biology) had a brilliant idea – why don’t we continue to use this format for the labs we do in class? Sometimes the obvious solution is right in front of your eyes!

Now we’ve revamped our prelab activities to have students methodically identify the different parts of the experiment they’ll be doing. The handout goes in their notebook, but you could just as easily have students handwrite the information in their notebooks to save paper.

The worksheet starts with a brief explanation of what they will be doing in the lab. We make sure the explanation contains enough information that students CAN identify all the parts of the experiment. Below the explanation is space to write. For example, we did a simple osmosis/dialysis tube lab. The worksheet we used is below:

Student sample for the osmosis lab

Students work together in their lab groups to identify all of the parts. While they’re working, I circulate through the room, listening for misconceptions and answering questions.

Since we started using this process, students have become more adept at understanding the different components of an experiment, as well as being able to watch for the results they expect (as opposed to trying to figure it out after the fact). When it’s time to graph their data, they can easily identify which variable goes on each axis, so I’ve seen much better graphs this semester. In addition, when they write their lab reports, it much easier for them to transfer the information from this worksheet.

Big Ideas About Experimental Design – An Interactive Activity

It feels obligatory to start off the school year with a lesson on The Scientific Method, which exists in the mind of students as a Thing. For many students, it’s a rigid, lockstep slog through a mythical world of how scientists work. And by the time they get to high school, they’ve definitely heard it several times.

Rather than spend a lot of time rehashing what they already knew, I wanted to tease out the important parts of experimental design in a more interactive manner. I also wanted to get my students up and moving around the classroom and getting to know each other.

Predictably, when I asked my students “How many of you have studied ‘the scientific method’ (and yes, I used air quotes)?” a good 90% of them raised their hands. I’m sure they thought I was going to launch into the usual scientific method lesson.

I’d prepared sheets of butcher paper ahead of time with four prompts written at the top: Hypothesis, Variables & constants, How to write the procedure, and Types of data. I put each piece of butcher paper at a separate table group and gave each student a pad of sticky notes. The instructions I gave them were to write down anything they knew about their prompt and stick it to the paper. I asked them to use a separate sticky note for each idea. Once they started writing, I gave them a few minutes to get all their ideas down on paper.

After a few minutes, I had groups rotate to another table (with their sticky notes). At the new table, I asked the groups to read the new prompt, read the ideas that were already on the paper, and add any new ideas to the paper. They could also correct any ideas that they thought were incorrect. We repeated this process until each group had visited all four tables.

Once they returned to their original group, I gave them a few minutes to read all of the ideas for their prompt. Then I asked each group to share with the class what they thought were the main ideas for their prompt. As they summarized, I wrote down all their ideas on the board. If I thought a group was missing something important, I first asked them a question to see if they could come up with the answer, then opened the question to the whole class if they couldn’t.

Since this was the second day of class, my students hadn’t brought in their notebooks yet. Rather than have them copy these notes, I created a handout for them to glue in once we got the notebooks set up. I collected the Big Ideas from all of my classes and compiled them into one handout that I called “Big Ideas About Experimental Design”.

All in all, this was a painless way to find out what students already knew about designing a scientific experiment. Although this doesn’t include everything, it created a launching pad to get the class started on our scientific explorations.

What does this mean? Teaching students to analyze graphs

Once students have learned how to correctly graph their data, the next step is for them to figure out what that graph is telling them about their data. In past years, I would talk to students about identifying trends, or looking at specific points on the graph, but I didn’t have a very robust lesson on graph analysis. That all changed in the summer of 2017 when my colleague and I went to the Stanford Academy for Excellence in Biology Teaching, a weeklong training offered by the Stanford Graduate School of Education Center to Support Excellence in Teaching (CSET) – whew, that’s a mouthful! The training incorporated a lot of information and strategies from the AP Biology Teacher Academy that was developed in partnership with BSCS, NABT, and HHMI Biointeractive. (If it’s offered in the future, consider doing this course – it was well worth it. I also participated in the year-long follow-up to earn graduate credit from Stanford.)

One of the lessons we learned was the BSCS Identify and Interpret (I2) Strategy. This strategy has students break down their analysis of a graph into two parts. The first is the Identify step, also known as “What I See”. In this step, students are identifying changes, trends or differences. The idea is that they are looking at the big picture, without a lot of specifics. They mark places on the graph where they see a particular change/trend and identify what the change or trend is. The second part is the “Interpret”, or “What It Means” step, where students assign meaning to their “What I See” observations. There is a final step in the BSCS strategy where students write a caption that summarizes the information and where students can demonstrate their understanding of the data. (Instead of having students write a caption, I had them write a supported conclusion.)

I did some direct instruction on the strategy, giving students the BSCS “What I See/What It Means” (WIS/WIM) examples, along with what they were supposed to do for each part of the strategy. Next, we looked at a sample graph – I had the graph projected on the board, so I started with an example where I identified a point, then wrote a WIS statement. I gave students a few minutes to mark their WIS points and make observations. Then I had students come up to the board, identify their WIS point, and wrote a WIS statement on the board. After all of the important points were identified, I wrote an example WIM statement for the WIS point I identified, then gave students time to come up with WIM statements for the WIS points we had on the board. Again, students came to the board and wrote WIM statements that corresponded to the WIM statements.

It seemed like they got it, so I gave students a handout with another graph for practice in the same class period. The handout* included a graph, a grid to write down their WIS and WIM statements, and a third page that gave them room to write a 3-4 sentence conclusion about how the presence or absence of soil fungus affected the growth of plants.

The WIS/WIM grid

When students finished their graph analysis, I collected the handouts. I wanted to give them a quick turnaround for feedback, so I graded them before the next class session. My feedback focused solely on their written conclusion. The assignment was worth 5 points, so I created a rubric and included student sample answers for each point value so students could see what each criteria looked like.

The basic rubric – see the handout for student sample answers

Honestly, the “What I See” step was the hardest part for my students. They could easily identify points on the graph that seemed to be important, but went right for explaining what it meant. Instead of saying “non-sterilized has a steeper slope compared to sterilized soil” for their WIS statement, they would say something like “plants grew better in non-sterilized soil than sterilized soil”. In general, though, their conclusions were pretty good, especially considering I hadn’t taught them anything about Claim-Evidence-Reasoning yet!

Later in the year, my colleague instructed students that the WIS statement should only identify a point/range on the X-axis and make an observation about what was happening at that point or range. She also instructed students that the WIM statement should describe how the X-axis affects the y-axis. I went back and had students add this information to their earlier notes, and this clarification seemed to help.

All told, this is a great strategy to help students learn how to thoroughly analyze their graphs. I think it helped them write solid conclusions, because they had identified specific parts of the graph that they could then incorporate into their conclusions. If I had reinforced the strategy more throughout the year, students would have been more comfortable with it. My goal for next year is to have a “What I See/What It Means” practice graph for each unit.

*When I print the handout for my students’ notebooks, I print it two sided, with two pages on each side. They can fold the handout in half to make a booklet that fits on one page in their composition book. My colleague joked about me making booklets, but when I taught in public school, I had to buy copy paper myself, so making booklets was an easy way to save paper.

Teaching basic graphing skills

One of the first lessons I teach each year is graphing. Now I don’t know about you, but for my students (9th grade), everything is a bar graph. EVERYTHING. I don’t know where this idea gets embedded in their brains, but it’s probably one of the hardest habits to break. I start early, reinforce it often, and grade consistently throughout the year. And, what do you know, by the end of last year, all of my students were using the appropriate type of graph!

For my biology students (regular level, 9th graders), I keep it simple. I’m only going to teach them to differentiate between line graphs, bar graphs, and pie charts. I’m teaching them the foundation and leaving it to other teachers (their math or future science teachers) to get more detailed, with concepts like scatterplots, lines of best fit, error bars, etc.

I’ve always taught scientific graphing, even before I was fully immersed in NGSS, but seeing how this fit into the Crosscutting Concepts (I’m going to abbreviate it as CCC from here) just reinforces the importance of graphing. The best fit of graphing in to the CCC is in Patterns, Cause and Effect, and Scale, Proportion and Quantity. (In addition, NSTA has prepared a great matrix of the CCC.) In the Grades 6-8 Patterns section, the matrix explicitly says that “graphs . . . can be used to identify patterns in data.” A well-plotted graph can show the relationship between a manipulated/independent variable and the responding/dependent variable. From there, students can infer cause and effect.

I have students take notes on graphing on a foldable that gets glued in their notebooks. During the presentation, we discuss what a “good” graph includes, and when to use each kind of graph.

Graphing foldable for student note-taking

There are a couple of useful acronyms that I give my students to help them remember how to properly graph their data. The first is “DRY MIX” – “DRY” means that the Dependent/Responding variable goes on the Y-axis, and “MIX” means that the Manipulated/Independent variable goes on the X-axis. I like this acronym because some textbooks or teachers will use the terms manipulated and responding to refer to variables, while others will use independent and dependent, so this acronym covers both of those bases.

The second acronym I give my students is “TAILS”. I wish I remembered where I first found this acronym, because it is so useful!

  • T stands for Title – I teach my students that their title should explain the relationship between the independent variable the the dependent variable. (I ask them to make a title that explicitly states the relationship – if they’re at a total loss, I give them a standard template of “Effect of -IV- on -DV-“.)
  • A stands for Axes – this incorporates the DRY MIX acronym to make sure the variables are on the correct axes.
  • I stands for Intervals – the number intervals on the axes should be evenly spaced.
  • L stands for Labels – the axes should be labeled with the variable and units, and there must be a key/legend if there are multiple lines or categories.
  • S stands for Scale – I tell students that their graph should take up as much space on the graph paper as possible, usually at least 2/3 or 3/4 of the space. No teeny-tiny graphs, and no graphs that go outside the boundaries of the grid (which means don’t draw your own lines at the top or right of the grid).

At the bottom of the foldable, I explain when each type of graph is appropriate. With line graphs, I struggled with how to explain it to students in a general enough way that they got it. “Tracking continuous changes in the independent variable” was confusing and not exactly correct. When I looked at the CCC, the light finally came on – a line graph is used to show the cause and effect relationship between the variables. To state it as a question, “As the scientist changes the independent variable, how does that affect the dependent variable?” I will still keep the “continuous data” part of it to help students see that if there is a continuum of the units for the independent variable, then a line graph is the best choice.

The categories of the TAILS acronym get incorporated into a 10-point grading rubric (which I won’t post because it’s not my own work product). Students get a copy of the grading rubric at the beginning of the year, and it is glued into the reference section of their science notebook. I also made a mini-rubric summary (just the categories and point values) that I print out – when I grade, I can just circle the point awarded for each category. I staple the mini-rubric to their paper so students can see where they lost points.

What I noticed last year was that by explicitly teaching graphing skills AND consistently using the graph grading rubric all year, students quickly developed their graphing skills. Most students were graphing at the 9.5-10 point level by the end of the first semester. The biggest challenges seemed to be in crafting a graph title, and in figuring out the intervals on the axes. By the end of the school year, when students were crunching data for their final projects, every student was using the appropriate graph for their data, and was correctly graphing their data.

A follow-up to the Agar Cell Size lab

In my recent post, “A new spin on the agar cell size lab”, I outlined my take on a common surface-area-to-volume-ratio activity. Since then, my students have turned in their lab assignments, so I wanted to follow up with how they did. Students were very successful at collecting raw data, but struggled with some of the calculations. I am going to make some modifications to my instructions to clarify a couple of things.

Recorded data – both BTB agar spheres were submerged in vinegar for 15 minutes
Each student group had a small sphere; I had the large sphere and shared that photo with students

One benefit of having a 1:1 device school is that I could easily share my photo of the large sphere with students. I wanted students to insert photos of both spheres into their document so they could complete the calculations at home without having to figure out where their photos were. I also had several students absent for this lab, so I could email them both photos – no need to have them come in later for a make-up lab because the only data they needed were the photographs.

Calculating the rate of diffusion

I had students calculate the rate of diffusion only for the large sphere. In a later question, students used that rate of diffusion to calculate how long it would take a molecule to completely diffuse into the center of each sphere (assuming the rate of diffusion to be constant for both spheres).

Once they made that comparative calculation, the final question asked them to summarize their findings, CER style. One of the main foci this year has been analyzing labs using the claim-evidence-reasoning practices. Students used their calculations to explain how changing the SA/V ratio affects a cell’s efficiency.

Evidence-based conclusion, CER style

By focusing on one small aspect of cell size, students were able to more clearly articulate why cells tend to be small. In addition, since the agar was pre-formed, there wasn’t the margin of error that results from students cutting their own agar shapes.

How does meiosis produce genetic diversity? A hands-on but simple modeling activity

It’s easy to say, but hard to visualize – “Meiosis produces genetic diversity through crossing over and independent assortment.” NGSS Standard HS-LS3-2 says (in relevant part), “Make and defend a claim based on evidence that inheritable genetic variations may result from new genetic combinations through meiosis.” Last week I went over this with my classes a couple of times – I gave a mini-lecture and had students take notes that included a description of crossing over and independent assortment. I provided them with a blank diagram that I drew with two pairs of homologous chromosomes, and had them illustrate crossing over first, and then independent assortment into gametes. We did the Build-a-Bird activity from the Genetic Science Learning Center with paper chromosomes. But after completing both those activities, students couldn’t quite make the jump from modeling the processes with just one or two chromosomes to understanding how this process happens with a larger number of chromosomes.

The word I kept repeating was “random”, and pointed out where we saw randomness in each of the activities. In the Build-a-Bird activity, after each student had completed crossing over, I had them look at other students’ chromosomes and notice that everyone had “crossed over” at different places. And I pointed out that it was random which of the four gametes they chose for their offspring. Still, based on the questions they were asking me, I felt like they weren’t quite getting it. I think part of the difficulty was because the activities we used had such a small number of chromosomes and genes.

I struggled to find something online that could show the concept, especially independent assortment, but without taking up another whole class period. I didn’t find anything, so I decided to put together something on my own. One of the things I especially liked about the Build-a-Bird activity was that students were physically modeling concepts on paper chromosomes, so that was my starting point. I created 12 chromosomes (because my largest class has 12 students ) of varying sizes, and spread 26 different “genes” over those chromosomes, with a variety of homozygous and heterozygous alleles. I printed them on colored paper so students could see the paternal and maternal chromosomes, and also so that when they modeled crossing over, it would be visually clear where it happened.

Replicated chromosomes prior to crossing over

Each student got one chromosome – I explained that the chromosomes had already been replicated, so each student got one tetrad. I instructed students to perform crossing over either one or two times, at anyplace on the chromosomes, as long as the crossing over was between non-sister chromatids. To “cross over”, students cut the non-sister chromatids at the same place, switched the cut pieces, then taped the chromosome back together. (To be honest, this activity doesn’t really show the randomness of crossing over since each student has a different chromosome. The Build-a-Bird activity is much better for this.) After they completed crossing over, I had students separate their tetrad into four separate chromatids, turn over their papers, and mix them up.

To demonstrate independent assortment, I set four large beakers on a table and told students that each beaker was going to represent one gamete. I instructed them to bring their chromatids and drop each one into a different beaker, in whatever order they wanted.

Four separate gametes, each containing twelve chromatids

Each table group got one beaker – I instructed them to place the chromosomes in order and tape them onto a piece of construction paper. They could then easily see that each of the gametes had a unique combination of alleles and of paternal and maternal chromosomes.

The final gametes

I had students write down the alleles in each gamete, then work with their table groups to discuss how this activity showed the randomness of crossing over, and the randomness of independent assortment. They wrote a brief reflection on their understanding of genetic diversity. As I circulated and listened to students discussing the prompts, they seemed to have a deeper understanding of how these two events worked together to create diversity in gametes.

About halfway through the first class, a lightbulb went off – if I have each of my classes do this activity, I can then create a display showing that one individual can create many unique gametes because of the huge number of possible crossing over events and the huge number of possible ways to sort chromatids into gametes. I ended up with 16 gametes, so I made the display below and hung it in the hallway outside my classrooms. (I float between two classrooms, so I wanted all of my classes to be able to see the display.)

One individual, many unique gametes

I think that the kinesthetic activity of putting each of the four chromatids into a separate “bucket” helped students understand the role of independent assortment in creating genetic diversity. The benefit of this activity (compared to Chromaseratops or similar activities) is that it was very quick – the whole activity took 15 minutes at most, and that it isolates one of the more confusing concepts. By demonstrating independent assortment with 12 chromosomes (and 26 genes), it’s not a great leap from there to understand why siblings are so different from each other.

4/22 edit: Here are the documents I created for this activity. Feel free to use them – they are set to “View only”, so you will need to download them. Modeling Meiosis and Gametogenesis worksheet; Chromosomes/tetrads; Chromosomes/parent.