Final answer:
The sex-linked disorder common among European royal families and traceable to Queen Victoria is hemophilia, an X-linked recessive blood-clotting disorder predominantly affecting males.
Step-by-step explanation:
The sex-linked disorder that was common among the royal families of Europe, with all affected males tracing their ancestry to Queen Victoria of England, is hemophilia. Hemophilia is a blood-clotting disorder that is inherited in an X-linked recessive pattern. This means that males only require one copy of the affected X chromosome to express the disorder, whereas females, who have two X chromosomes, must receive the mutated gene from both parents to be affected, making the condition much rarer in females. Queen Victoria was a carrier of the trait, and through her daughters, the disorder was transmitted to various royal families across Europe. Females in an X-linked recessive inheritance structure have a 50 percent chance of being carriers if their mother is a carrier and their father is unaffected. In contrast, there is a 50 percent chance that a son will be affected by the disorder under the same parentage. It's the manifestation of this inheritance pattern that accounts for the prevalence of hemophilia among the male descendants of European royal families.