Rumple fathered Bae before he became the dark one. Therefore the “dark genes” shouldn’t really affect Bae because he was conceived when Rumple was a normal man.
that was my initial reaction but when you stop and think about what Jane said and how cancer works it makes sense. Cancer can be caused by a mutation in your genes. Like when something in the environment causes a change and you genes become altered in the cancerous cell and that cell multiplies like crazy. Certain chemicals and radiation can do that. Sometimes the gene is already there and somewhat dormant and than triggered by something. Think the breast cancer gene.
Keeper of Hook and Emma's smoldering first kiss, a certain Pirate's sauciness, the Evil Queen's snarkiness, Grumpy's gruffness and a drop of true love to make it all go down smooth.