Sister chromatid exchanges in Down syndromes and normal human beings.

The BrdU-Giemsa method was used to analyze the frequency of SCEs in a group of five Down syndromes and in a group of five normal human beings. In total 25 second mitoses were scored for SCEs in each individual. Although Down syndromes exhibited a tendency to have higher rates of exchange than normal human beings the analysis of variance showed that these differences were not statistically significant. On the other hand, significant differences were observed in the rates of SCEs between individuals within each group. This variability may reflect inter-individual differences in the efficiency of the mechanisms involved in the production of exchanges. The frequency of SCEs in blood cultures in probably the average of the rates exhibited by two or more lymphocyte sub-populations with different sensitivities to BrdU. Hence, the variability in the rate of exchange between different cultures of the same individual probably arises by changes in the percentage of cells in the second mitosis deriving from each lymphocyte sub-population.