Chromosomes are distinguishable by size, centromere position, satellites, staining patterns, and DNA probes. Dark-staining DNA, called heterochromatin, maintains the chromosome's structure. Polyploid cells have extra chromosome sets, and aneuploids have extra or missing individual chromosomes. Sex chromosome aneuploidy is less severe than autosomal aneuploidy. Nondisjunction, an uneven division of chromosomes in meiosis, causes aneuploidy.
Chromosomal rearrangements disrupt meiotic pairing, which can delete or duplicate genes.
An inversion flips gene order, affecting the phenotype if it disrupts a vital gene. A Robertsonian translocation fuses the long arms of two nonhomologs. In a reciprocal translocation two nonhomologs exchange parts. Translocation carriers have reproductive problems because they produce some unbalanced gametes as a result of the unequal division of chromosome regions during meiosis. Fragile X syndrome causes mental retardation and is associated with a fragile site on the X chromosome.