So why does modern US society still go along the one drop rule, a southern ( Democrat) slaver construct( I remember , as a child, being shocked on watching Show Boat where Ava Gardner was hunted down for having a drop of negroid blood in her)?

One should have DNA studies carried out and one is whatever the genetic study says one is....that goes for pronouns as well.

In the Domincan Republic, the opposite occurs, if one has one drop of white, one is white.

There is only one pigmentary scale ...the Fitzpatrick dermatological scale....and it has squat to do with race.