Any trait that showed up in the males only would be considered to be sex-linked to the X chromosome. This is because a male being XY would show the effect and the females who were carriers being XX would obviously have the normal ( non mutant ) X chromosome too, which dominates the mutated one. A classic example of X linked ( sex linked) traits is Haemophilia who are nearly always male. But a female haemophiliac is possible but could only be produced from having a haemophiliac father and a carrier mother ( A rare combination) and therefore inheriting two copies of the mutated recessive x "bleeding" gene. Hope this helps.