THE FREQUENCY AND DISTRIBUTION OF SISTER CHROMATID EXCHANGES (SCES) IN THE INDIVIDUAL CHROMOSOMES OF HUMAN KA¬RYOTYPE

The difference in the number of SCEs found per cell in 50 mitoses of control subjects when evaluated from photomicrographs or karyotypes is small but sig­nificant (p The mean value of 9.5 SCEs per mitosis found in the control group was in no way significantly different from mean values of the other groups, i.e. those consisting of pathological karyotypes and lymphoblastoid cell lines (p >0.05). High frequency of SCEs (20%) was found in the centromeric region of chro­mosomes after the 2nd and 3rd divisions in BUdR medium (6). The distribution of SCEs both observed and expected on the basis of the rela­tive length of chromosomal groups in the karyotype with results of χ2 test is shown in Table II. The changes were similar in all observed groups. The only exception were B group chromosomes of the lymphoblastoid ceil lines, where the number of breakpoints increased significantly (p We preferred to evaluate the non-banded chromosomes (G-banding considerably interferes with the accuracy of SCE calculation), the regions of break points are only roughly delineated. Even so it is clear that some regions are more often involved in SCEs than others. This is the case especially with regions 1q1, 3q2, 4q2, 8c and 16c. SCEs seem to be preferentially located on G-negative bands, as mentioned also by Morad , Jonasson and Lindsten (3)