192 Cellular Events Produced by Ionizing Radiations / 10 : 4 



By and large, the different characteristics are segregated during 

 meiosis according to the member of the homologous pair on which they 

 are located. However, occasionally pieces of the chromosomes break 

 off during meiosis. The broken pieces then rejoin the same homol- 

 ogous pair, but often a part of chromosome A will join the remainder of 

 A' and vice versa. This breaking and re-forming is known as crossing 

 over. This is hard to observe in animals which reproduce slowly, such 

 as the large mammals, because crossing over is a rare event. However, 

 in fruit flies, wasps, microorganisms, and viruses, the rate of reproduction 

 is so large that the frequencies of crossing over between two loci can be 

 accurately measured. Figure 4 shows a possible crossing over during 

 meiosis. 



b c d e f g h i j a b c d e f g' h' i' j' 



-•- 



a b c d e f g h i j a' b' c' d' e' f g h i j 



Before After 



Figure 4. One type of crossover. Letters show locations along 

 chromosomes. This is schematic only; most chromosomes are 

 not straight lines and are always twisted when crossovers 

 occur. Dot indicates centromere. For instance, a might 

 represent blue eyes, a' brown; and g might represent tall, g' 

 short. Then the offspring with no crossover would always 

 have blue eyes and be tall. With the crossovers shown, blue 

 eyes can occur with short. After H. J. Mueller, Chapter 7 in 

 Radiation Biology, Vol. 1, Part 1, A. Hollaender, ed., (New 

 York: McGraw-Hill Book Company, Inc., 1954). 



One action of all ionizing radiation and ultraviolet light is to increase 

 the frequency of crossing over of parts of homologous chromosomes. 

 Far from being undesirable, this is a beneficial effect increasing the minor 

 variations within the population. Ultraviolet irradiation strongly favors 

 crossing over as opposed to other genetic changes to be discussed. 

 Radiation-induced crossovers have increased man's knowledge of 

 genetic mechanisms. 



4. Visible Cellular Effects 



The cellular changes resulting from ionizing radiation are essentially 

 independent of the type of irradiation, provided similar ionization occurs. 



