Genome sequence variations largely determine the diversity of phenotypes observed in humans and all organisms. ALDH2 is the human gene encoding mitochondrial aldehyde dehydrogenase, responsible for a key step in alcohol metabolism. A protein-coding, single-nucleotide polymorphism in ALDH2 affects alcohol tolerance and leads to Asian flush and increased alcohol-associated cancer risk in individuals carrying the defective variant. Students are introduced to genetic traits and variation, sequence alignment to identify similarities and variants in genes, and bioinformatic tools that they will use to analyze functionality of two ALDH2 variants. Next, they will be given a patient’s partial genomic sequence data for ALDH2, make a diagnosis based on the knowledge they have accumulated, and act as genetic counselors to advise the patient.