Determination of the DNA content of human chromosomes by flow cytometry

The mean relative DN A content of each human chromosome was calculated from How karyotypes of ethidium bromide stained chromosomes obtained from healthy, normal individuals. These values were found to correlate closely with previously published data obtained by photometric scanning of stained, lixed chromosomes. Calculations of the normal variation in DNA content of each human chromosome indicated that chromosomes 1, 9, 16, and Y (chromosomes with large centric heterochromatic regions) were the most variahlc. followed by the acrocentrics, 13, 14, 15, 21, and 22. Chromosomes 2, 3, 18, and 19 were also found to vary significantly in DNA content. Chromosomes from a number of subjects with extreme hctcromorphisms were flow karyotyped to obtain an estimate of the extent of variation in DNA content of each chromosome. The greatest dilTcrcnce between extreme variants was found for chromosome I (which differed by 0.82% of the total genomic DNA). followed by 16 and 9. The largest Y-chromosome variant was 85.9% bigger than the smallest. The precise karyotype analysis produced by flow cytometry resolved many differences between chromosome homologs. including some that cannot be readily distinguished cytogenctically. The implications of these findings for detection of chromosome abnormalities by flow karyotype analysis are discussed.