Which statement accurately describes sex-linked inheritance?

• A. Traits are never expressed in males
• B. The Y chromosome carries most sex-linked genes
• C. Traits on the X or Y chromosome can be expressed differently in males and females
• D. All sex-linked traits are autosomal-like

Answer: (C)

Sex-linked inheritance involves genes located on the sex chromosomes, mainly the X chromosome in humans. Because males have one X and one Y, they are hemizygous for X-linked genes, so any allele on their single X is expressed without a second X to mask it. Females have two X chromosomes, so they can be homozygous or heterozygous for X-linked genes, and expression can be influenced by X-inactivation, which creates a mosaic pattern of activity in different cells. This combination means traits on the X (and to a lesser extent the Y) can show up differently in males and females. For example, X-linked recessive conditions like color blindness tend to manifest in males who have only one X with the mutant allele, whereas females would require two copies to be affected and often act as carriers. Note that the Y carries relatively few genes, so it does not account for most sex-linked traits.