Effects of age, sex and genes on sister chromatid exchange

Sister chromatid exchange (SCE) was studied in cultured lymphocytes from a limited series of 21 like-sexed twin pairs; 11 monozygotic (MZ) and 10 dizygotic (DZ) pairs. The 18 subjects, who were between 57 and 61 years old, had an SCE mean value (mean) of 8.0 whereas the 24 subjects between 33 and 39 years of age had a mean of 6.8. The difference was statistically significant (P less than 0.001). The effect of age appeared to be present in both sexes. No significant difference was found between females (mean = 7.3) and males (mean = 7.5), nor between smokers (mean = 7.3) and non-smokers (mean = 7.4). Drug users had a slightly higher mean (mean = 7.9) than non-users (mean = 7.0) (P less than 0.05). This trend was found in each age group. The within-pair variance was slightly higher in DZ than in MZ pairs. The difference was not significant. We conclude that genetic factors are probably not a major source of subject variation in SCE mean value.