This paper presents a realtime music composition system based on a theme and variation approach. The system uses MIDI and allows up to sixteen instruments to play together. Each instrument requires input of short MIDI files containing musical sequences. Operations inspired by genetic algorithms generate new sequences. Opposed to true, bit-level genetic algorithms the operations modify concrete note properties. This approach facilitates the generation of new sequences within the context intended by the user. Based on the input files the system establishes a list of filters for the purpose of selection. Selection becomes a straight-forward task that affords no user intervention. Scheduling inserts an entire selected sequence into an output queue at a time.