Chromosomal changes associated with changes in development.

In the past there has been a tendency to dwell on aspects of chromosomes which stress constancy of structure, number and content; even to the extent of dismissing exceptions as 'aberrations' or 'oddities'. It is now becoming clear that genomes can be quite plastic, and that structural changes to chromosomes are an important and often necessary part of normal differentiation and development. Elimination of whole chromosome sets or defined portions of genomes is not uncommon and selective gene amplification has been shown to be part of normal development in both protozoa and higher organisms. Chromosomal rearrangements are now a well-documented feature of normal development of, for example, B- and T-lymphocytes and trypanosomes. Transposable elements, whose mobility may not be part of normal developmental processes, can have marked effects on development if their transposition takes them to developmentally important genes. This article reviews some of the structural changes that occur during normal development, and discusses some of the consequences for development when the mechanisms which bring about these rearrangements go wrong.