Lactose intolerance, caused by a lack of persistent lactase enzyme expression, is a trait commonly observed in adult humans, with varying geographic prevalence depending on dietary habits in different cultures. This case study follows a diverse group of undergraduate students on their quest to understand why some humans are able to digest lactose and others cannot. Students are asked to use quantitative reasoning to explore the geographic distribution of the trait of lactose intolerance as well as the enzymatic activity of the lactase enzyme. Students then explore eukaryotic gene regulation and how single nucleotide polymorphisms (SNPs) can affect the expression of the lactase gene. Students are asked to correlate genotypic data with phenotypic observations. Finally, students explore the effect of environmental factors on the evolution and geographic distribution of the lactase persistence trait by generating concept maps. The case was developed for introductory biology courses up to introductory molecular biology courses at the college level and emphasizes quantitative reasoning as well as integrating different concepts using the topic of lactose intolerance.