Correct Answer: B. 2/3rd of Mendelian disorders are autosomal dominant
The distribution of Mendelian disorders in human populations follows a well-documented epidemiological pattern. Approximately 2/3rd (67%) of all Mendelian disorders are autosomal dominant, while only 1/3rd (33%) are autosomal recessive. This skewed distribution reflects fundamental principles of inheritance and disease manifestation. Autosomal dominant disorders require only a single mutant allele for phenotypic expression, making them more readily apparent in pedigrees and easier to transmit across generations. Classic examples in Indian populations include familial hypercholesterolemia, achondroplasia, Marfan syndrome, and hereditary spherocytosis. X-linked and Y-linked disorders together constitute a small fraction (<5%) of all Mendelian disorders. The 2/3 : 1/3 ratio emerges because dominant mutations are more likely to be detected clinically (heterozygotes show the phenotype), whereas recessive disorders require homozygosity or compound heterozygosity—a much rarer scenario in outbred populations. This epidemiological fact is critical for understanding disease burden in clinical genetics and genetic counseling in Indian medical practice, where consanguinity increases recessive disorder frequency but does not alter the overall Mendelian distribution significantly.
Why the other options are wrong
A. Every 5th patient has an X-linked recessive disorder — This is wrong because X-linked recessive disorders account for only ~5–7% of all Mendelian disorders, not 20% (1 in 5). The statement confuses population prevalence with Mendelian inheritance distribution. X-linked disorders are rare in the overall Mendelian spectrum; they are overrepresented only in specific populations (e.g., haemophilia in certain families) or in male-only presentations. NBE trap: students may conflate the high visibility of X-linked disorders in pedigrees with their actual frequency. C. Every 10th patient has a Y-linked disorder — This is wrong because Y-linked disorders are extremely rare (<1% of Mendelian disorders) and affect only males. The statement vastly overestimates Y-linked disease frequency. Y-linked inheritance is restricted to traits on the non-recombining region of the Y chromosome (e.g., male sex determination, some cases of male infertility). This option is a distractor designed to test whether students understand that Y-linked disorders are exceptionally uncommon in clinical practice and epidemiological surveys. D. 1/3rd of Mendelian disorders is autosomal recessive — This is wrong because while 1/3rd of Mendelian disorders ARE autosomal recessive, the statement uses singular 'is' instead of plural 'are,' making it grammatically incorrect and semantically ambiguous. More critically, this option is a partial truth designed to trap students who know the 2:1 ratio but misread the question stem. The correct statement should be '1/3rd of Mendelian disorders are autosomal recessive,' but even then, option B is the more complete and clinically relevant answer.
High-Yield Facts
- 2/3 : 1/3 ratio — Approximately 67% of Mendelian disorders are autosomal dominant; 33% are autosomal recessive; <5% are X-linked or Y-linked combined.
- Autosomal dominant disorders require only one mutant allele for expression; heterozygotes manifest the phenotype, making them more detectable in pedigrees and populations.
- Autosomal recessive disorders require homozygosity or compound heterozygosity; much rarer in outbred populations but more common in consanguineous marriages (common in India).
- X-linked recessive disorders (e.g., haemophilia A/B, colour blindness) affect primarily males; carrier females rarely show symptoms unless skewed X-inactivation occurs.
- Y-linked disorders are exceptionally rare; limited to non-recombining Y chromosome regions; examples include male sex determination and some infertility cases.
Mnemonics
2:1 Mendelian Rule 2/3 Dominant : 1/3 Recessive — Two-thirds of all Mendelian disorders are autosomal dominant (one mutant allele = disease), one-third are autosomal recessive (two mutant alleles needed). X-linked and Y-linked are negligible (<5% combined). Why Dominant > Recessive H = Heterozygote shows disease — Dominant mutations are expressed in heterozygotes, so they are clinically visible and easily inherited. Recessive mutations hide in carriers, requiring rare homozygosity to manifest. Hence dominant disorders are more common in the population.
NBE Trap
NBE pairs the 2/3 ratio with grammatically flawed or partially true statements (like option D: "1/3rd is" instead of "are") to trap students who know the concept but misread the question. The trap exploits the fact that students may recognize the 1/3 recessive fraction and select it without noticing the grammatical error or realizing that option B is the more complete and clinically relevant answer.
Clinical Pearl
In Indian genetic counseling practice, the 2:1 ratio helps clinicians quickly estimate recurrence risk: a family with an autosomal dominant disorder has a 50% recurrence risk per child, while autosomal recessive disorders (more common in consanguineous marriages) have a 25% recurrence risk. This ratio is foundational for risk stratification in prenatal diagnosis and family screening programs across India.
_Reference: Robbins and Cotran Pathologic Basis of Disease, Ch. 5 (Genetic Disorders); Harrison's Principles of Internal Medicine, Ch. 64 (Genetic Counseling)_