Which of the following best defines a sex-linked trait?

• A. A trait controlled by genes on an autosome
• B. A trait determined by environmental factors
• C. A trait controlled by genes on the sex chromosome
• D. A trait determined solely by mitochondrial genes

Answer: (C)

Sex-linked traits are defined by genes located on the sex chromosome, usually the X chromosome in humans. Because males have only one X (and one Y), a recessive allele on the X will be expressed in a male if there isn’t a second X to mask it, while females have two X chromosomes and typically require two copies of a recessive allele to show the trait. This makes the pattern of inheritance distinct from autosomal traits, which are on non-sex chromosomes and affect both sexes more evenly. The other options describe traits not tied to the sex chromosome (environmental factors, autosomal genes, or mitochondrial genes with maternal inheritance), so they don’t define what a sex-linked trait is.