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< 0.01).
Therefore, the data described have been obtained from karyo­types only.

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<0.01). In this particular chromosomal group the breakpoints
leading to exchanges were distributed pro­portionally along the whole length of all chromosomes.
Significant difference was found not only as against the expected number of
SCEs but also as against the control group. The increased number of breakpoints
on the long chromosomes was naturally matched by
their decrease in small chromosomes (5).

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)