In this HHMI Biointeractive hands-on activity, students use photos of stickleback fish to analyze the results of genetic crosses between fish with different traits. Students are introduced to the activity through a suggested video: "Students should watch the short film Evolving Switches, Evolving Bodies before doing this activity. You may consider showing the film up to the point where Dr. David Kingsley describes how genetic crosses using stickleback fish were done (4 min. and 45 sec. after the start of the film). You could stop the film at that point, have the students complete this activity, and then finish showing the film." p.2 of Teacher Materials. Students are asked to classify fish according to the presence or absence of pelvic fins, use Punnett squares to predict frequencies of genotypes and phenotypes, make a claim about the mode of inheritance and to evaluate evidence to justify their claim. Teachers can extend the activity by asking students to use chi-square to evaluate data and provide students with the opportunity to experiment with ideas about determining genotypes with their own test crosses. This activity is supported by a video, a Teacher Guide, a Student Handout, and the required fish pictures which may all be downloaded from the website.