When a plant carries a transgenic copy of an endogenous gene, both genes may be silenced. This 'cosuppression' can occur not only within individual calls, but also in distant cells through an agent that apparently moves through the plant's phloem.
Smyth, D. R. (1997). Gene silencing: Cosuppression at a distance. Current Biology, 7(12). https://doi.org/10.1016/s0960-9822(06)00407-6