Why are X-linked inheritance patterns more likely to affect males?

• A. Males have two X chromosomes; recessive allele from both must be present.
• B. Males have only one X; a single recessive allele expresses phenotype.
• C. Y chromosome carries disease.
• D. Autosomal genes involved.

Answer: (B)

X-linked genes sit on the X chromosome, and males have only one X (plus a Y). For X-linked recessive traits, a male needs only one recessive allele on that single X to show the trait, because there isn’t a second X with a potentially dominant allele to mask it. Females have two X chromosomes, so a recessive allele on one X can be masked by a normal allele on the other X, and only those females with recessive alleles on both Xs express the trait. The Y chromosome can’t compensate for X-linked genes, so these traits tend to show up more in males.