Biology teachers consider basic Mendelian genetics to be value-free, objective science, immune to misinterpretation and misuse. It may thus come as a surprise to learn that in the early days of genetics a cornerstone of genetics education, the dihybrid cross, was employed to support claims of the racial superiority of whites over blacks and to provide a “scientific” rationale for laws prohibiting interracial marriages. In 1917 the prominent eugenicist Charles B. Davenport warned of the danger of “disharmonious combinations” of physical and behavioral traits in the second generation of “wide race crosses,” equivalent to the F 2 generation of a dihybrid cross. He tried and failed to find data to support his arguments in a study of the mixed-race inhabitants of Jamaica. Davenport's analysis was deeply flawed, especially by the racist assumptions underlying this work. Although these events occurred a century ago, biology teachers may still be able to use this regrettable episode as an example of how even the most basic science may be misapplied by those with a social or political agenda.