Methods and compositions for detecting an allelic form of a target. In an exemplary method, partitions may be created that collectively contain at least one first allelic form and a second allelic form of a target. Each partition may contain (i) a same probe capable of binding specifically to each of the first and second allelic forms of the target and (ii) a competitor configured to bind selectively to the second allelic form relative to the first allelic form and to block binding of the probe to the second allelic form. The first allelic form of the target may be amplified in the partitions.A signal may be detected from a label of the probe while the label is contained by the partitions. A number of partitions that are positive (or negative) for the at least one first allelic form may be determined based on the signal.