STUDIES OF THE INHERITANCE OF HUMAN RIBOSOMAL DNA VARIANTS DETECTED IN 2-DIMENSIONAL SEPARATIONS OF GENOMIC RESTRICTION FRAGMENTS

We have investigated the variation in human ribosomal DNA repeat units as revealed in two-dimensional electrophoretic separations of genomic restriction fragments that were end-labeled at NotI cleavage sites. T he transcribed portion of the ribosomal DNA results in similar to 20 l abeled fragments visible on each gel as multicopy spots. We have mappe d these spots to the sequences responsible for their appearance on the gels, based on their migration positions and direct sequencing of spo ts, and describe several previously unreported sources of variation. B y studying mother/father/child families we gained information on how m uch of the between-repeats variation is due to differences between and within repeat arrays on homologous chromosomes. Two instances in whic h a child exhibited more copies of a particular fragment than were pre sent in the parents are described and hypothesized to be due to events such as multiple unequal sister-chromatid exchanges or gene conversio ns.