Final answer:
Sex-linked traits are unique because they are located on the sex chromosomes and are expressed differently in males and females due to the difference in their sex chromosome composition (XY in males and XX in females).
Step-by-step explanation:
Sex-linked traits are unique due to two important factors. Firstly, they are located on genes on the sex chromosomes, rather than on the autosomes. This means that the inheritance of these traits is directly linked to the sex chromosomes - X and Y. The second factor is that since males (XY) and females (XX) have different sex chromosomes, sex-linked traits are expressed differently between the sexes. For example, traits due to genes on the X chromosome are considered X-linked and can manifest differently in males and females because males have only one copy of the X chromosome, whereas females have two.