The purpose of this research was to determine whether teaching argumentation to high school biology students improved their argumentation skills, informal reasoning, and genetics understanding. Using a quasi-experiment with mixed methods of data collection, five teachers participated in professional learning on argumentation and socioscientific issues and then explicitly taught argumentation skills in a genetics context. Using a written survey, the experimental group of students (n = 133) improved significantly more in their argumentation skills (p < 0.001), ability to use rational informal reasoning (p < 0.001), and genetics understanding (p < 0.001) than the control group of students (n = 160) who studied the same genetics topic without being taught argumentation skills.