Preface

It has been a well known fact for over 60 years that the genome undergoes rearrangements, or what seem to be a general scrambling of the order of the genome. In the salivary glands of Drosophila, a phenomenon of chromosomes doubling in thickness during mitosis has been noticed. This appears to be two homologs (identical copies of a chromosome segment created during cell division) that have glued together somehow. The chromosomes have an observable pattern of bands perpendicular to their length, which were studies since the 1920's. This pattern is characteristic of a species. However, at times one can find two individuals of the species who show different patterns of these bands; usually the differences appear to be segment reversals along the pattern of bands.