Characterization of Constitutive Marker Chromosomes in Humans

A marker chromosome is one that is morphologically different from any normal chromosome with its origin not being readily discernible by classical cytogenetic techniques. Marker chromosomes in neoplasia often represent the products of complex rearrangements and will not be discussed in this chapter. From a large series of 377,357 karyotyped amniocentesis specimens in a multicenter survey, an unbiased estimate of the frequency of de novo constitutional marker chromosomes was 1 in 2500 (1). The overall population frequency will be higher than this since markers identified as i(18p) or i(12p), and markers that were familial, were excluded. Since the proportion of familial markers is in the vicinity of 40% (2), the actual population frequency of marker chromosomes is about 1 in 1200.