In mammals, which have an XX/XY system of sex determination, the X chromosome has many more genes than the Y chromosome. This explains the inheritance of such conditions as colour blindness and haemophilia. They are caused by recessive alleles, and usually do not show the condition in females. This is because those females carrying the mutant are most likely to be heterozygous for the allele. However, if the mutant allele is carried by a male, the condition will develop, because his X chromosome has the mutant allele, but his Y chromosome has no allele at that locus.
Sex-linked characters controlled by genes on sex chromosomes are not the only kind of sex-related characters.
Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body. Even in a homozygous female the condition may not be expressed fully. Example: baldness in humans.
These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes. Examples: female sterility in Drosophila; and polymorphism in insects, especially in relation to mimicry. Closely linked genes on autosomes called "supergenes" are often responsible for the latter.
Sex linkage Facts for Kids. Kiddle Encyclopedia.