What’s Downstream? A Set of Classroom Exercises to Help Students Understand Recessive Epistasis †

A hypothetical yeast enzymatic pathway regulated by a hypothetical signaling pathway. (A) Enzymatic pathway alone (see text for context). A, B, and C are small molecules. Heavy arrows represent enzymatic reactions, and light arrows represent expression of the genes that encode the corresponding enzymes in the pathway. This pathway is controlled by the presence or absence of sucralose. (B) The same enzymatic pathway (lower line), intersecting with a signaling pathway (blue line) that is controlled by the presence or absence of neotame. In the signaling pathway, R, S, and T are proteins with the functions shown, encoded by Genes 3, 4, and 5 respectively. Light arrows indicate gene expression, but here, heavy arrows represent regulatory interactions: pointed arrows indicate activation, and the blunt arrow indicates inhibition. For example, in the presence of both sucralose and neotame, if the receptor R is activated by neotame binding, it activates the protein S. If S is active, it inactivates the transcription factor T, which is otherwise active and is required to activate transcription of Gene 2.

Epistasis pretest and posttest questions. Correct answers are underlined.

Clicker questions from in-class activity and follow-up. Correct answers are underlined and percentages of correct responses from students in the developmental biology class described in the discussion are shown.

In-class application questions. (A) This question asks students to draw possible pathways of gene interaction, given the phenotype of C. elegans mutants. (B) Clicker question in which students must choose the correct pathway based on additional evidence presented. Student performance on this question is shown for the individual vote, and following peer discussion. Correct answers are underlined.

Performance (% correct) on the pretest and posttest questions. Pretest performance is represented by the blue bars and posttest performance by the red bars. For two posttest questions (2 and 3), students discussed their answer and re-voted (green bars). Average performance on the posttest, both before and after re-vote, is significantly higher than on the pretest (paired t-test, p < 0.05).