A 35-year-old man with X-linked hypophosphatemic rickets (XLHR) due to a PHEX gene mutation presents to the genetics clinic with his wife for family planning. The pedigree pattern shows the inheritance characteristic marked as **B** — all daughters of affected fathers are affected, while no sons inherit the condition from affected fathers. Which of the following best explains why this inheritance pattern is diagnostic of X-linked dominant transmission rather than autosomal dominant inheritance?