Which of the following is an X-Iinked disorder?

Explanation

## Correct Answer: C. Color blindness Color blindness (red-green color blindness) is an **X-linked recessive disorder** caused by mutations in genes encoding red and green cone photopigments located on the X chromosome. The discriminating feature is the chromosomal location: the opsin genes for red and green color vision are clustered on Xq28, making this the only X-linked condition among the options. Males (XY) require only one mutant allele to express the phenotype, while females (XX) are typically carriers unless homozygous. Prevalence in Indian males is approximately 8%, making it the most common X-linked disorder in the population. The condition manifests as inability to distinguish red from green hues, detected by Ishihara color plates in clinical practice. Unlike the autosomal recessive disorders listed (cystic fibrosis, thalassemia, sickle cell anemia), color blindness shows characteristic X-linked inheritance: affected males have carrier daughters, and affected females have affected sons. This inheritance pattern is the key diagnostic feature tested in NEET PG genetics questions. ## Why the other options are wrong **A. Cystic fibrosis** — Cystic fibrosis is an **autosomal recessive disorder** caused by mutations in the CFTR gene on chromosome 7, not the X chromosome. It presents with pancreatic insufficiency, chronic lung infections, and elevated sweat chloride (>60 mEq/L diagnostic). The inheritance pattern shows affected individuals with carrier parents, not the male-predominance and carrier mother pattern of X-linked disorders. This is a common distractor because it is a well-known genetic disorder in pediatric practice. **B. Thalassemia** — Thalassemia is an **autosomal recessive disorder** affecting hemoglobin synthesis, with genes located on chromosome 11. In India, thalassemia major is endemic in certain populations (Mediterranean, Middle Eastern, Southeast Asian ancestry), but inheritance is autosomal—both parents must be carriers for affected offspring. The condition causes hemolytic anemia, hepatosplenomegaly, and bone deformities. NBE pairs thalassemia with color blindness to test understanding of chromosomal location, not just phenotypic severity. **D. Sickle cell anemia** — Sickle cell anemia is an **autosomal recessive disorder** caused by a point mutation (glutamic acid → valine at position 6) in the β-globin gene on chromosome 11. It is endemic in tribal populations of central India (Madhya Pradesh, Chhattisgarh). Inheritance requires two mutant alleles; heterozygotes are sickle trait carriers. The vaso-occlusive crises and hemolytic anemia are autosomal recessive manifestations, not X-linked. This option tests whether students confuse genetic severity with chromosomal inheritance. ## High-Yield Facts - **Red-green color blindness** is X-linked recessive; located at Xq28 (opsin genes). - **Prevalence in Indian males**: ~8%; females rarely affected unless homozygous. - **Inheritance pattern**: Affected males → carrier daughters → affected grandsons (skips generations in females). - **Diagnosis**: Ishihara color plates (pseudoisochromatic plates); affected individuals miss numbers embedded in dot patterns. - **Cystic fibrosis, thalassemia, sickle cell anemia** are all **autosomal recessive** (chromosome 7, 11, 11 respectively). - **X-linked vs. autosomal**: Only X-linked shows male predominance, carrier mothers, and affected-to-carrier-daughter transmission. ## Mnemonics **X-linked RED flag** **R**ed-green color blindness, **E**mophilia, **D**uchenne muscular dystrophy = classic X-linked recessive. Use when differentiating X-linked from autosomal disorders. **Autosomal Recessive 'CFTS'** **C**ystic fibrosis, **F**riedreich ataxia, **T**halassemia, **S**ickle cell = common autosomal recessive. Helps exclude these when X-linked is the answer. ## NBE Trap NBE pairs color blindness with three severe autosomal recessive disorders (CF, thalassemia, SCA) to test whether students confuse disease severity or prevalence with chromosomal inheritance. Students may incorrectly choose thalassemia or sickle cell anemia because these are more clinically severe or endemic in India, missing the chromosomal location distinction. ## Clinical Pearl In Indian pediatric practice, color blindness is often discovered incidentally during school vision screening or when a boy cannot identify colors in classroom activities. Genetic counseling for carrier mothers is important: all daughters of affected males are carriers, and sons of carrier mothers have a 50% risk of being affected—a pattern that helps confirm X-linked inheritance at the bedside. _Reference: OP Ghai (Pediatrics) Ch. 2 (Genetics); Robbins (Pathology) Ch. 5 (Genetic Disorders)_