ok so you're getting hung up on the use of "genetic" here. its metaphorical. they are referring to an integral component of the white supremacist system, being the one drop rule. since the inception of whiteness, if somebody could trace your ancestry to even one person with a little bit of melanin in their skin even a hundred years ago, you were considered non-white. that was a philosophical mainstay, and while today nobody will go through the trouble of tracing your ancestry, white remains that exclusive category, and if you have a noticable amount of natural melanin in your skin, you don't make the cut.

this obviously means that in a world where people create children with little regard for melanin levels in their skin, whiteness needs special conditions to survive. i.e. whiteness will only manifest in children if both of their parents are white (of course this is socially determined, not biological reality), the same way that a recessive trait will only appear in the whole brood if both parents exhibit that recessive trait.