Say that many members of the royal family have hemophilia. Is this due to genes, or environment? If it is genes, you can try to track down who has the gene, and not marry any of those people to your current monarch. If it is something in the royal water supply, you can track that down. If you say "It's both!", you are unlikely to solve the problem.

If it is both then you are more likely to solve the problem by saying "It's both" than by saying "it's one!"