An analysis of the inherited pattern of C-band heterochromatin has been made in five pedigrees containing a total of 33 offspring that were available for analysis. The majority of variants were found to be inherited; however, at least seven of the 99 variants were not present in either parent, and an additional seven differed from the parental variant by either a morphological change or the appearance of mosaicism. It is believed that the polymorphism of human constitutive heterochromatin arises from a mismatching of the repetitive DNA sequences contained in these regions with subsequent unequal crossing over. Further, the observed mosaic patterns provide suggestive evidence that such an event occurs in somatic cells as well as during meiosis.