I have frequently struggled to help students connect the events of meiosis with the adaptive value of sexual reproduction; it’s hard to get students to look away from the stages of meiosis to see the “big picture” of genetic variability.
So I was interested to read a recent article by Dorit Eliyahu in The American Biology Teacher (full reference below). Eliyahu’s exercise leads students through a simulation of meiosis and reproduction in a fictional “reptilobird,” using paper chromosomes. Each student is issued a set of six chromosomes, including two pairs of autosomes and a pair of sex chromosomes. Each chromosome has 4 to 11 gene loci. The alleles for each gene are marked, and a key is provided for seven of the genes. For those seven genes, students can infer a reptilobird’s phenotype from its genotype.
I immediately liked that the students actually “do” crossing over between homologous chromosomes, at points that the students choose themselves. Already I saw an improvement over my own lab exercise, which uses paper chromosome cutouts but ignores crossing over.
The best part was yet to come, though. The students use their chromosomes to make gametes, then use those gametes to make and describe a baby reptilobird. In my current lab exercise students make a baby and characterize its features too, but the Eliyahu exercise goes one step farther. Students actually draw their reptilobirds and tape them to the board, so the whole class can see the variation that meiosis produces. Genius! As a bonus, the exercise incorporates clever questions that reinforce ideas of Mendelian genetics, codominance, incomplete dominance, sex linkage, gene linkage, epistasis, and pleiotropy. Bravo!
I tried this exercise for the first time, along with my husband/colleague Doug Gaffin, in his class Action Center a couple of weeks ago. About 35 students participated. It worked extremely well, with one exception: Students took a lot of artistic liberties with their reptilobirds. That turned out to be a problem, because a lot of the variation among the drawings came from the feather shapes, body colors, rainbows, and other decorations that were not specified in the instructions. (The photo at the top of this post reveals a small sample of our students’ artistic talents.)
We therefore decided to improve the process by adding a bank of cutout “reptilobird parts” for students to color and tape onto their reptilobird bodies, which should standardize the offspring phenotypes a bit more. We also changed the function of one gene: The X-linked phenotype of “color blindness” is impossible to draw onto the reptilobird body, so we changed that character to “beak color.”
The article in The American Biology Teacher clearly lays out the instructions for the exercise, and Eliyahu’s handouts are freely available. However, I made some modifications to incorporate the “body part” bank and to adjust the content of the handouts to better suit our class needs; I also created a modified list of materials to assemble before each class. Please contact me via the comments section of this blog if you would like me to send my revised materials to you.
In the meantime, thank you to Dorit Eliyahu! We will definitely be chasing baby reptilobirds around the lab next fall.
Reference: Eliyahu, Dorit. 2014. “Chromoseratops meiosus”: A simple, two-phase exercise to represent the connection between meiosis and increased genetic diversity. The American Biology Teacher 76 (1): 53-56.
Update, 10/14/14: We used the reptilobird activity for the first time in last week’s lab. As you can see from the photo at right, they were much more consistent in format, making it much easier to see the variation among the birds. This photo shows some of the birds scattered on the floor. In the classroom, however, my TAs arranged the four birds that corresponded to the four possible parental phenotypes along the top of the bulletin board. The students pinned their own birds below. The huge number of offspring phenotypes were a great illustration of the potential variation that comes from meiosis.
Only one thing was missing: an easy way to tell male from female. The next time we do this activity, the sheet with the reptilobird body will sport two new additions: large male and female symbols, which students can color in to indicate the sex of their bird. Other than that, I am very satisfied with this activity and can highly recommend it as a way to connect meiosis with genetic variation and natural selection.
Update 2, 10/19/15: We did this lab again last week, and we made an important discovery. We thought it would be a good idea to save student time by having our undergraduate assistant cut out all of the chromosomes ahead of time and deliver them to students in coin envelopes. BIG MISTAKE! The students no longer understood what was homologous to what, mass confusion ensued, and the activity did not go well. Things went much better in the next lab session, when students had to cut out their own chromosomes as usual.
Also, if you want to incorporate an evolutionary connection into this activity, check out my followup blog post from June 2015.