Is scoliosis dominant or a recessive gene?

There is no known single gene that causes scoliosis..
The fact that there is genetic influence is widely accepted, and it's more common in females than males, but that's all that's known.
But judging by the fact that it's much more common in females, if I had to take a guess, I'd say it's a Sex-Linked recessive allele that plays into it.