In a plant, gene Q codes for a certain enzyme, and the unlinked gene P codes for a regulatory protein that is needed to bind to the regulatory region of Q in order for transcription to occur. Given recessive null mutations in both genes, you carry out a series of Mendelian crosses.
What proportion of the individuals of a dihybrid self cross will lack enzyme Q activity? Assume that lack of enzyme activity is not lethal. Express your answer as a reduced fraction.