Inherited retinal diseases cause progressive vision loss and usually follow a single inheritance pattern, either dominant (one altered gene copy) or recessive (two altered copies). However, some “dual-inheritance” genes can cause disease through both modes, posing unique challenges for genetic counselling. Analysing data from Portugal’s national registry of over 1,100 patients, we identified nine such genes, accounting for 12% of all genetic diagnoses and affecting 102 families. Our findings reveal that the specific location and type of genetic change, not just the gene itself, determines how the disease is inherited, shaping symptoms, severity and prognosis. These insights carry important implications for genetic counselling and the design of future gene therapies. (https://jmg.bmj.com/content/early/2026/06/19/jmg-2026-111618)