Locus Heterogeneity — Retinitis Pigmentosa

A 32-year-old Indian man presents with progressive night blindness over the past 5 years, followed by gradual loss of peripheral vision. Fundoscopy reveals waxy pallor of the optic disc, attenuated retinal arterioles, and characteristic bone-spicule pigmentation in the mid-periphery. ERG shows reduced scotopic responses. Genetic testing identifies a mutation in the RHO gene (rhodopsin). His younger brother has an identical clinical presentation but carries a mutation in the ABCA4 gene. The structure marked **A** in the diagram represents the concept that explains why both brothers have the same clinical phenotype despite carrying mutations in different genes. Which of the following best describes this genetic principle?