An extensive study has been made of the transport and magnetic properties of the Y1-xPrxBa2Cu3Oy system for a range of compositions (0 less-than-or-equal-to x less-than-or-equal-to 0.6, y almost-equal-to 7). Although the critical temperature T(c) decreases monotonically and smoothly with increasing x, the material shows qualitative changes in properties at a threshold Pr concentration x(cl) almost-equal-to 0.46 before T(c) becomes zero at another threshold x(c2) almost-equal-to 0.58. As x increases towards x(cl), a slight upward deviation from linearity in the resistivity vs. temperature curve begins to appear in a temperature region just above T(c), which suddenly becomes prominent when x exceeds x(cl). The diamagnetic signals, observed both in the field-cooled and zero-field-cooled branches, decrease with increasing x but the decrease is particularly steep when x increases through x(cl). For all the samples studied, small but significant diamagnetism is found to persist up to a temperature region well above T(c) determined from measurements of resistivity. These observations are discussed in terms of a two-dimensional percolation model in which it is assumed that two Pr ions, when they occupy neighbouring rare earth sites, localize a hole on the oxygen sites between them, locally destroying the superconductivity.