chromosome aberrations in Tradescantia 731 



exchanges are primarily two hit, the data on photoelectrons can be used 

 to set a lower limit to the distance from a given break at which the second 

 break must occur. Calculations based on physically equivalent neutron 

 and X-ray doses, e.g., at 50 r, indicate that 23 protons and 303 photo- 

 electrons pass through a nucleus of diameter 12 n and set these limits as 

 <1.3 and >0.9 jjl, respectively. Thus the average distance over which 

 reunion occurs is concluded to be approximately 1 n. Independent 

 estimates derived from the ratio of interstitial deletions to asymmetrical 

 exchanges and from the most frequent size of interstitial deletions are in 

 agreement with this calculation. 



PROPORTION OF BREAKS THAT UNDERGO RESTITUTION 



Evidence has been presented in the preceding section that reunion is 

 not at random, and that breaks separated by distances greater than 

 approximately 1 /z usually do not undergo reunion. From this evidence, 

 plus the results of the intensity experiments, it can be concluded that 

 the majority of breaks undergo restitution. Further general evidence 

 for restitution comes from data on the frequencies of different kinds of 

 breaks observed in individual cells. For example, as pointed out by 

 Catcheside (1948), it seems clear that chromatid exchanges and chromatid 

 breaks arise from a similar initial event, a chromatid break, reunion 

 occurring in one case and not in the other. If, in fact, chromatid 

 exchanges do arise in this manner from reunion of two chromatid breaks, 

 and if all chromatid breaks not participating in such reunions were pre- 

 served, it is apparent that the occurrence of chromatid exchanges should 

 affect the distribution of surviving chromatid breaks. However, in cells 

 with exchanges, there is no evidence of an excess of odd as compared with 

 even numbers of surviving chromatid breaks. Lea (1946) and Catche- 

 side, Lea, and Thoday (1946b) have developed several quantitative 

 methods for calculating from the number of observed aberrations of 

 various types the number of breaks primarily produced, and hence the 

 proportion of breaks which restitute. These include (1) a comparison of 

 the frequencies of incomplete reunion and restitution in various chromatid 

 aberration types; (2) a consideration of the departure at high doses from 

 the dose-squared relation for X-ray-induced exchanges; (3) comparisons 

 of expected and observed break frequencies with fast neutrons; (4) com- 

 parisons, based on the observed ratio of chromatid exchanges to chro- 

 matid-isochromatid exchanges, and of chromatid to isochromatid breaks, 

 of the relative proportions of primary chromatid and isochromatid breaks 

 which persist; and (5) a consideration of the relative frequencies of 

 chromatid and isochromatid breaks. All these calculations agree in 

 indicating that only a minority of the breaks primarily produced in 

 Tradescantia microspores are scored and that the majority restitute and 

 escape recognition. The estimates of the fraction (f) of unjoinable 



