Trait positive
From what Daniel, Jonas, and Sarah discovered in the box left by Bree's father, it is apparent that Bree has an inordinate amount of ribozymes. According to Daniel, "ribozymes are like molecular scissors that cut DNA . . . scientists seeking the origin of life have produced ribozymes that are actually capable of catalyzing their own synthesis. Translation: They somehow make life.
The Order invests an extrordinary amount of time cultivating these girls' blood, because the Elders use it to seek eternal life, as confirmed by LaRezisto in Help Us!.
Fans can conclude that girls asked to perform the Ceremony have a high level of ribozymes (such as Bree, Kate, or Emma), and as such, are "Trait Positive." Any other girls with a normal or low level of ribozymes are "Trait Negative," and therefore, undesirable to the Order.
