When asked about protein, students often mention meat, protein bars, and protein's role in building muscles. Many students are not aware of the most basic function of protein: linking genes and traits. Because of its importance in molecular genetics, protein function is included in the life sciences section of the "Next Generation Science Standards" (NGSS Lead States 2013). Students should be able to explain how the structure of DNA determines the structure and function of proteins, including essential structural, signaling, transport, and catalytic activities in cells. Proteins' role in trait-producing mechanisms is challenging to grasp, as most students connect genotype and phenotype by explaining that genotypes "give" phenotypes (i.e., genes directly determine traits), completely bypassing the role of proteins in the process. This article describes a 10th-grade biology unit (six or seven instructional days) the authors developed on this topic that addresses the driving question: "How do Siamese cats get their coloration?" It asks students to make explicit connections among genes, proteins, and traits. The unit has been taught in both urban and suburban schools.