The males's genome contain XY chromosomes, while female's genome have XX chromosome. If a man has X-linked allele it will be expressed (e.g. the diseases will appear). If a woman has one X-linked allele she will be the carrier (but will not develop the disease). Daughter will inherit one X chromosome from mother and another from father. So, if a man has X-linked disease, all of his daughters will become carriers of the disease.
