Which one of the following is caused by the expression of a recessive gene present on sex chromosome ?

A recessive gene on a sex chromosome typically refers to X‑linked recessive inheritance. Human sex chromosomes differ (females XX, males XY), so males have only one X and express recessive alleles present there without a second copy to mask them [1]. In classical Mendelian terms a recessive trait needs two copies to express in autosomes, but for X‑linked traits a single mutated allele on the male’s lone X is sufficient for expression [2]. Duchenne and Becker forms of muscular dystrophy are caused by mutations in the DMD gene on the X chromosome and follow X‑linked recessive inheritance, primarily affecting boys while females are usually carriers [3]. Therefore, muscular dystrophy is the correct choice.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 8: Heredity > 8.2.4 Sex Determination > p. 132
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 8: Heredity > What you have learnt > p. 133
3. [3] https://www.ninds.nih.gov/health-information/disorders/muscular-dystrophy