Males have one X chromosome and one Y chromosome whereas females have 2 X chromosomes. Hence females have 2 genes one on each of the X chromosome controlling the respective phenotype(appearance).Males only have one gene on the X chromosome controlling the phenotype because the Y chromosome is not known to control any phenotype linked to the X chromosome. Hence traits controlled by recessive alleles on the X chromosome are more common in males than in females. For a recessive trait to show in a female, she should inherit two X chromosomes that contain the recessive genes.Where as for a male to inherit a recessive trait, he only has to inherit one X chromosome which contains the recessive gene. Hope this helps!