A 28-year-old man presents with progressive night blindness since age 12, followed by gradual peripheral vision loss. Fundoscopy reveals bone-spicule retinal pigmentation, attenuated arterioles, and waxy optic disc pallor. Full-field ERG shows markedly reduced scotopic responses. Genetic testing identifies a heterozygous mutation in the RHO gene (autosomal dominant inheritance), while his unaffected cousin with identical clinical presentation carries a biallelic mutation in USH2A (autosomal recessive). The phenomenon illustrated by **A** in the diagram—where the same clinical phenotype of retinitis pigmentosa results from mutations in different genes—is best described as which of the following?