the methodology and results of epidemiological studies of the effects of preconception diagnostic x - rays of the abdomen on chromosome segregation in humans are described . 
 many studies have been conducted in a number of different countries . 
 the vast majority show the same positive , though not significant , trend to increased nondisjunction among the offspring of irradiated women . the results of the various studies , however , can not be pooled because of differing methodologies used . 
 a worldwide co - operative project with standardized methodology is recommended . 
 such a study should identify the parental origin of the nondisjunctional event before etiological factors are investigated . 
 abnormal chromosome segregation during mitotic division has been inducted experimentally by the in vitro exposure of human lymphocytes to a low dose of 50 r gamma irradiation . 
 first meiotic nondisjunction has been successfully induced by whole body exposure of female mice to a low dose of radiation . 
 further experiments are being conducted to try to induce abnormal segregation during second meiotic division . 
 because of difficulties encountered in trying to estimate total gonad doses resulting from differing techniques employed by radiologists and other health personnel , no attempt has been made to estimate the doubling dose nor minimum safe dose regarding the effects of radiation on chromosome segregation in humans . 
 the question of time - related repair of the mechanism involved in chromosome segregation is raised.imagesfigure 1 .