2: Once you take an actual genetics class you'll know it's often far more complicated that.
3: What makes you think the gene is recessive? Because OR6A2, the cilantro gene, is actually "dominant," though other factors can suppress its expression in this matter. See point #2. Who knows what this theoretical grapefruit gene is...
I grew up eating it too. I thought maybe I didn't like the ones from our tree, but I got one from the supermarket once and it was the same flavor. I understand grapefruit is supposed to be fairly bitter but to me it tastes more like bile.
You would think the parents that love it would feed it to their kid, but kids are weird about food so maybe OP didn't eat anything but bananas and peanut butter.