Genetic testing can be life-changing for hereditary cancer or cardiac conditions, but most tests offer probabilistic risk estimates rather than deterministic predictions. Peter Attia explains when genetic testing genuinely changes clinical decisions versus when measuring phenotype directly—through blood work and imaging—provides more actionable insight, and how to avoid accumulating genetic data without gaining clarity. Most genetic tests reveal probabilistic risk shifts, not guaranteed outcomes; 50% of Alzheimer's patients lack APOE4 and many APOE4 carriers never develop dementia, demonstrating that genotype does not determine phenotype.