I have my doubts that a sweet orange is a direct parent of grapefruit. Pomelos are not as bitter as grapefruit (they taste like tropical fruit punch, basically). Neither are oranges. Why would they combine into something more bitter than either parent?
I could believe that grapefruit and pomelo are closely related, however, but I think the other parent is probably something bitter, and likely not an orange fruit, either.
Location: SW Idaho, USA
USDA hardiness zone: 6
Elevation: 2,260 feet Profile post