Chromosome inversion: A condition in which a chromosome segment is clipped out, turned upside down, and reinserted back into the chromosome. A chromosome inversion can be inherited from one or both parents, or it may be a mutation that appears in a child whose family has no history of chromosome inversion. An inversion can be 'balanced,' meaning that it has all the genes that are present in a normal chromosome; or it can be 'unbalanced,' meaning that genes have been deleted (lost) or duplicated. A balanced inversion causes no problems. An unbalanced inversion is often associated with problems such as developmental delay, mental retardation, and birth defects.