A 35-year-old Indian male presents with progressive bitemporal hair recession and vertex thinning over the past 10 years. His father and paternal grandfather were both bald, but his mother and maternal grandmother have full scalp hair despite carrying the same allele. Genetic testing confirms an autosomal dominant susceptibility variant. The inheritance pattern shown in the pedigree is marked as **A** in the diagram. Which of the following BEST explains why this trait is expressed in males but NOT in females who carry the same allele?

Explanation

## Why option 1 is correct Sex-limited inheritance refers to autosomal traits whose **phenotypic expression is restricted to one sex** even though the gene is autosomal and both sexes can carry and transmit the allele. Male-pattern baldness (androgenetic alopecia) is the classic example: the susceptibility allele is autosomal dominant, but expression requires sustained high androgen (dihydrotestosterone/DHT) stimulation of follicles. Females, having low circulating androgens, do not express the bald phenotype unless hyperandrogenic states (PCOS, postmenopausal) develop. The pathophysiology involves variants in the androgen receptor (AR) and 20p11 locus that increase follicular sensitivity to DHT, leading to follicular miniaturization and progressive hair loss in males. This is the defining feature of sex-limited inheritance: **the gene is autosomal, but phenotypic expression is restricted to one sex due to hormonal/anatomic requirements** (Robbins Basic Pathology, 11th ed., Ch. 6; Thompson & Thompson Genetics in Medicine, 8th ed., Ch. 7). ## Why each distractor is wrong - **Option 2 (X-linked recessive)**: X-linked traits show a characteristic skipped-generation pattern (affected grandfather → carrier mother → affected grandson). Males are hemizygous and express the trait; heterozygous females are typically unaffected but are carriers. This pedigree shows autosomal transmission (affected males have affected fathers, not just carrier mothers), ruling out X-linked inheritance. - **Option 3 (Y-linked dominant)**: Y-linked traits are transmitted exclusively from father to son, and **all sons of affected fathers are affected**. However, the pedigree shows that not all males necessarily inherit the trait (depending on the mother's genotype in an autosomal pattern), and females can carry the allele and transmit it to sons. Y-linked inheritance would not fit this pattern. - **Option 4 (Sex-influenced inheritance)**: Sex-influenced traits (e.g., gout, hemochromatosis) allow **both sexes to express the phenotype, but with different penetrance or severity**. In contrast, sex-limited inheritance restricts phenotypic expression to **only one sex**. The key distinction is that in sex-limited traits, females do not express the phenotype at all (unless hormonal changes occur), whereas in sex-influenced traits, both sexes can be affected. **High-Yield:** Sex-limited = autosomal gene, expression in ONE sex only (due to hormonal/anatomic requirement); sex-influenced = autosomal gene, expression in BOTH sexes but unequal penetrance. Male-pattern baldness is the prototypical sex-limited trait. [cite: Robbins Basic Pathology, 11th ed., Ch. 6; Thompson & Thompson Genetics in Medicine, 8th ed., Ch. 7]