As with an autosomal dominant trait, the X-linked dominant must be present in every generation of the affected lineage. The genotype of an individual with an X-linked dominant trait can be inferred directly from inspection of the phenotype.
In a marriage between an affected male (AY) (I-1) and an unaffected woman (aa), the father's A will be passed to all daughters, as will the mother's a, such that all daughters are heterozygotes (Aa). All sons will receive a Y from their father and an a from their mother, and will be unaffected (aY). An affected heterozygous woman (Aa) (II-2) who marries an unaffected man (aY) will pass the A allele on to half of her children, including sons (AY) and daughters (Aa) in equal proportions.

    Note that affected women are almost always heterozygous (Aa). If an affected woman were homozygous (AA), all of her children (male and female) would be affected.