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?

Explanation

## Why option 1 is correct Locus heterogeneity is the hallmark genetic principle demonstrated by retinitis pigmentosa (RP). The anchor **A** in the diagram represents this concept: the SAME PHENOTYPE (progressive night blindness → peripheral visual field loss → bone-spicule pigmentation → eventual blindness) is caused by mutations at DIFFERENT GENETIC LOCI (different genes). In this case, the proband's RHO mutation and his brother's ABCA4 mutation are in entirely different genes, yet both produce clinically indistinguishable RP. This occurs because >80 genes have been identified in RP (autosomal dominant: RHO, PRPH2, RP1; autosomal recessive: ABCA4, RPE65, EYS, USH2A; X-linked: RPGR, RP2), and all converge on photoreceptor function and survival pathways. The clinical phenotype is similar regardless of which gene is mutated because the final common pathway is photoreceptor degeneration. This is the defining feature of locus heterogeneity and is a critical concept for understanding genetic diseases in clinical practice (Hartong et al. Lancet 2006; Thompson & Thompson Genetics in Medicine 8th ed., Ch. 7-8). ## Why each distractor is wrong - **Option 2 (Allelic heterogeneity)**: While allelic heterogeneity does occur in RP (e.g., multiple different mutations within the RHO gene itself), the question specifically states that the two brothers carry mutations in DIFFERENT GENES (RHO vs. ABCA4). Allelic heterogeneity refers to different mutations within the SAME gene, not different genes. This is the definition of locus heterogeneity, not allelic heterogeneity. - **Option 3 (Genetic anticipation with trinucleotide repeat expansion)**: Anticipation refers to progressive worsening of a genetic disease across successive generations due to expansion of unstable trinucleotide repeats (e.g., Huntington disease, fragile X syndrome). RP is not caused by trinucleotide repeat expansions, and the question does not describe worsening across generations—only progressive worsening within each individual's lifetime. This concept is not relevant to the genetic basis of RP. - **Option 4 (Mitochondrial heteroplasmy)**: Heteroplasmy refers to the coexistence of mutant and wild-type mitochondrial DNA within the same cell or individual, leading to variable phenotypic expression. While mitochondrial inheritance is rare in RP, the question describes autosomal inheritance (both brothers affected, suggesting autosomal dominant or recessive inheritance). Heteroplasmy does not explain why mutations in different nuclear genes (RHO and ABCA4) produce the same phenotype. **High-Yield:** SAME disease + DIFFERENT GENES = LOCUS heterogeneity (RP); SAME GENE + DIFFERENT MUTATIONS = ALLELIC heterogeneity (CFTR). [cite: Hartong et al. Lancet 2006; Cutting GR Nat Rev Genet 2015; Thompson & Thompson Genetics in Medicine 8th ed., Ch. 7-8]