TYPES AND FREQUENCIES OF CHROMOSOMAL ABERRATIONS INDUCED BY X-RAYS

Abstract

Previous analyses of X-ray induced chromosomal aberrations in Tradescentia microspores have led to the following conclusions. Either one or both of the sister chromatids at prophase can be broken by a single "hit." Exchanges or fusions between chromatids of different chromosomes are the result of two independent "hits," as are the ring and dicentric chromosomes induced during the resting stage. When the time of irradiation is constant for different X-ray doses the aberration frequency increases as the square of the dosage, but when X-ray intensity is constant the exponent of the dosage curve equation is less than two. The effectiveness of the time-intensity factor suggests that both breakage and fusion may occur during the time of exposure. Fractional dosage experiments have shown that most breaks undergo either restitution or produce chromosome aberrations within about an hour after irradiation (Sax, 1938, 1940).