Can molecular genetics make a boolean variable from a continuous variable?
Positive feedbacks can be one alternative. Positive feedbacks exhibit bistability and can therefore adopt one of the two stable states depending on the initial condition. A famous example of a positive feedback switch would be that between cI and Cro in λ-phage, which repress each other. Positive feedbacks also display hysteresis: if the state of the system is dependent on an inducing ligand X such that system attains state S2 at high levels of the ligand, then to switch from S2 to S1 the concentration of X would have to be reduced below the level that was required for the S1 to S2 switch; and vice-versa. In other words, the system tries to remain in its current state.

If the transcription factors that constitute the genetic positive feedback have a co-operative behaviour then you can expect a steep switching curve. 

For details see this review.