6o GENETIC VARIATIONS 



general effect on the organisms as a whole, and this effect is 

 injurious. They distinctly weaken the constitution of the 

 individuals in which they exist. The result is that the 

 mutated individuals lack resistance to severe conditions, and 

 have a shorter life and a higher mortality rate than those not 

 mutated. It is a common observation that individuals show- 

 ing the mutated characteristics are difficult to keep alive; 

 specially favorable conditions must be supplied or they perish. 

 Indeed, a large proportion of the mutations that occur cause 

 a complete failure of development, unless the mutated gene is 

 accompanied by an unmutated gene of the same type. Such 

 are known as lethal mutations. One is surprised, when he 

 comes to study the occurrence of mutations, to discover that 

 the commonest types are these lethal mutations; a large pro- 

 portion of our knowledge is based on these changes that cause 

 death. Often when one gene of a pair is thus affected by a 

 lethal mutation, while the other is normal, the individuals 

 are weak and abnormal in structure and function. If both 

 genes of the pair are affected by the mutation, the individual 

 "dies before it is born." 



There occur gene mutations whose effects, both special and 

 general, are so slight that from examination of the individuals 

 affected by them, it is difficult to determine whether they are 

 harmful or not, although as a rule the general impression they 

 give is one of slight abnormality or loss. But here the correct 

 interpretation is aided by certain extremely ingenious 

 experiments made possible by the action of radiation on 

 chromosomes and genes. The results of these experiments are 

 illuminating; we must examine them.^ 



As will be recalled, normally there are in the cells of any 

 individual two genes of each type, forming a pair. That is, 

 there are present two "doses" of the material represented by 

 the gene. By the use of radiations it is possible to obtain indi- 



