Which statement best describes a sex-linked trait?

A sex-linked trait is specifically associated with genes located on the sex chromosomes, which are responsible for determining an organism's sex. In many species, including humans, these sex chromosomes are typically referred to as X and Y. The traits expressed by genes on these chromosomes can show different patterns of inheritance depending on whether an individual is male or female, often leading to differences in how traits are expressed across sexes.

For example, conditions like hemophilia and color blindness are well-known sex-linked traits that predominantly affect males due to their inheritance patterns from the X chromosome. Males have one X and one Y chromosome, and thus only require one copy of a recessive allele on the X chromosome to express these traits, whereas females have two X chromosomes, typically requiring two copies to express the trait.

This distinction reflects the underlying genetic mechanisms of inheritance, where traits linked to autosomes (non-sex chromosomes) do not exhibit these sex-dependent expressions. Thus, the defining feature of sex-linked traits is their association with genes on sex chromosomes.