Why gene therapy cannot cure dominant disease?
You should bear in mind that the transferred normal gene is not a replacement of the defective gene. It is used to compensate for the defective gene. If the normal gene is dominant over disease-causing gene, the effect of the disease-causing gene will be masked.
If that is the case, the allele for the genetic defect cannot be dominant, because the dominant allele will still expressed even if a normal gene is introduced.