The invention relates to novel genetically modified plants and methods or materials, such as polynucleotides, expression cassettes, or vectors for producing the same. Moreover, the invention relates to altering the content of cannabinoids in plants and to medical compositions derived from such plants. In particular embodiments, the present invention relates to cannabis plants having modified expression of tetrahydrocannabinolic acid (THCA) synthase and methods of modifying the amount of delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) in cannabis by modifying expression of THCA synthase.