Inversion, pericentric chromosome: A basic type of chromosome rearrangement in which a segment that includes the centromere (and so is pericentric) has been snipped out of a chromosome, turned through 180 degrees (inverted), and inserted back into its original location in chromosome. The feature that makes it pericentric is that both breaks are on either side of the centromere (the point at which the chromosome attaches to the spindle).
An inversion can be "balanced", meaning that it has all the genes present in the normal uninverted chromosome. Or an inversion can be "unbalanced", meaning that genes been have deleted (lost) or duplicated.
A balanced inversion in a child causes no problems. An unbalanced inversion is abnormal and is often associated with problems such as development delay (and later, mental retardation) (substitute intellectual disability anywhere that mental retardation appears in glossary) and multiple congenital anomalies (birth defects).