Race has been a longstanding topic in the biology textbook curriculum. Yet, there appears to be no research investigating whether the treatment of race in modern biology textbooks impacts how students conceptualize race. In the present study, a double-blind field experiment employing mixed-methods is used to investigate the impact of textbook-based genetics learning on essentialist conceptions of race amongst adolescents. The study was carried out in an eighth grade classroom in a California Bay Area School. Students recruited for the study (N = 43) read either a racialized or a non-racialized textbook passage on human genetic diseases and completed a reading comprehension assessment. After a short distracting task they responded to items in two different race conception instruments. Controlling for race, gender, age, prior race-conceptions, and reading comprehension, statistically significant effects were observed on both race conception instruments by treatment. Students in the racialized condition exhibited stronger essentialist conceptions of race than students in the non-racialized condition. Additionally, an exploratory analysis indicated that an understanding of Mendelian heredity moderated the observed treatment effects. The findings of the present study tentatively suggest that textbook-based instruction in school biology can inadvertently reinforce essentialist conceptions of race that underlie racial bias. Implications for teaching and research are discussed.