Analysis of human chromosomal variants by quantitative electron microscopy. I. Group D chromosome with giant satellites.

HE so-called “normal” variations in the human karyotype found in clinically Tnormal individuals have been useful for linkage studies as chromosome markers (e.g., No. 1 and No. 16 chromosomes) (DONAHUE et al. 1968; MAGENIS, HECHT and LOVIEN 1970). The old order Amish in the United States are an inbred group that has been extensively studied genetically ( MCKUSICK, HOSTETLER and EGELAND 1964). During chromosome screening of Amish males for patrilinear variation in the length of the Y chromosome (BORGAONKAR et al. 1969), a D group chromosome was found to have a giant satellite knob (inset, Figure 1) . Extensive cytogenetic studies are in progress, and the data collected so far indicate that this giant-satellite carrying marker D chromosome is present in about 10% of the Amish in Lancaster County, Pa. Autoradiographic studies indicate that the marker chromosome may be a number 14 chromosome since it is labeled more heavily in the region proximal to the centromere (MCKUSICK 1969). Karyotyping of single human chromosomes by determinations of dry mass from electron micrographs has recently become possible (BAHR and GOLOMB 1971 ) and allow for rather precise grouping and evaluation of individual marker chromosomes. This technique is applied to the giant-satellite carrying large acrocentric chromosome in this study. Our study is the first in a series of detailed analyses of abnormal human chromosomes in which quantitative electron microscopy demonstrates the capacity of rendering new information in two orders of magnitude below the resolution of the light microscope.