10 : 5/ Cellular Events Produced by Ionizing Radiations 197 



produced in cells, the genetic effects are not very different for X rays, 

 gamma rays, beta rays, protons, and so on. Even neutrons and ultra- 

 violet irradiation give rise to qualitatively similar genetic effects, although 

 comparing their dosages in terms of ion pairs is not very meaningful. 

 Genetic changes are explained in terms of alterations of one or more 

 chromosomes. 



Specific places along the chromosomes are associated with final body 

 characteristics such as height, eye color, and number of fingers. These 

 spots are called genes. Along each chromosome there are a large number 

 of such genes. However, along a homologous pair of chromosomes, the 

 homologous genes control the same characteristics. Thus, in a human, 

 with 24 pairs of chromosomes per body cell, each pair controls a given 

 set of characteristics. In a highly inbred population, both chromo- 

 somes of the pair will usually be the same, but in normal populations 

 the two chromosomes usually will contain many different genes. The 

 dominant gene will determine the body characteristic. The recessive 

 gene, though not altering body form, may be transmitted genetically. 



It was discovered first with the mold neurospera that each gene 

 apparently controlled one enzyme. (Enzymes are biological catalysts 

 of a protein nature which control the rate of most chemical and physical 

 processes in living cells. They are discussed more fully in Chapters 17 

 and 18.) This idea led to the hypothesis of a one gene-one enzyme 

 relationship. Because the idea of the gene was a somewhat fuzzy one, 

 genes are now often defined biochemically as the part of a chromosome 

 associated with a given enzyme. 



Further studies of crossovers, particularly in neurospera and viruses, 

 but also in the fruit fly, drosophila, have shown that even this definition 

 of the gene — that is, the part of the chromosome associated with one 

 enzyme — may be misleading. Each enzyme is a protein made up of 

 amino acids ; changes in very small regions along a chromosome, perhaps 

 in pieces 20 A long, can alter one amino acid in an enzyme. However, 

 it is not customary to call this small piece a gene. 



When any of these tiny regions along the chromosome is altered, the 

 genetic character transmitted will be changed. Such changes are 

 referred to as mutations. Most mutations are recessive; that is, they 

 are carried along and reproduced in the chromosomes without changing 

 the body form until descendants occur in which both chromosomes have 

 this mutation. Most mutations are also lethal; that is, the progeny, 

 both of whose chromosomes have this mutation, either fail to form as 

 embryos or else do not reach maturity. A few mutations, perhaps one 

 in 10,000, are desirable in that they lead to a characteristic favoring the 

 survival of the species. 



The frequency of mutation in bacteria, paramecia, fruit flies, 



