https://www.forbes.com/sites/quora/2017/08/16/why-does-grapefruit-taste-bitter-to-some-people-but-not-others/#443e7a6f60d5

Why Does Grapefruit Taste Bitter To Some People, But Not Others?

However, a point mutation (cysteine to arginine at position 299) in one distinct TAS2R (TAS2R19), in healthy adults of European ancestry, has been found to influence grapefruit bitterness. Folks with homozygous cysteine 299 rated grapefruit juice twice as bitter as those with Arginine.

EDIT: I think that this is the study that is being discussed:

https://pmc.ncbi.nlm.nih.gov/articles/PMC3038275/