GENETIC VARIATIONS 345 



tated genes may be brought together into the same pairs, 

 in one individual (compare figure 61) ; thereupon the effect 

 of all these mutations suddenly becomes manifested in that 

 individual. Since most mutations are harmful in their ef- 

 fects, this individual will be weak and defective, perhaps in 

 several different ways. This is the reason why inbreeding is 

 harmful; if there were no gene mutations, apparently there 

 would be no disadvantage in the mating of close relatives. 

 One of the chief results of biparental reproduction — what 

 might be called one of its main functions — is that it largely 

 nullifies the effect of harmful mutations. 



In view of the fact that mutations may thus remain long 

 hidden, it is fortunate for purposes of study of the occur- 

 rence of mutations that in many organisms there is a series 

 of genes, in the X-chromosome of one of the sexes, that are 

 not paired but single. In the best known organisms it is the 

 male that has the single X-chromosome. If in the fertilized 

 egg, or in the cells from which the ovum is derived, a muta- 

 tion occurs in one of the genes of an X-chromosome, and 

 this passes into a male, the effects of the mutation are at 

 once manifested in the characteristics of that individual. 

 Thus the time and place of the occurrence of gene mutations 

 has been studied most extensively in the genes of the X- 

 chromosome. In recent years however an elaborate and ade- 

 quate technique has been devised for bringing to light al- 

 most at once the effect of mutations m the genes of any of 

 the chromosomes of Drosophila. In this way much has been 

 learned as to the time and place of the occurrence of muta- 

 tions, and as to the agents causing them. 



It has been discovered that a gene mutation may occur in 

 any cell of the body, at any period in the life of the organ- 

 ism. The mutation occurs in but one single cell. If this single 

 cell is the fertilized egg, then all the cells of the body de- 

 veloped from that egg will carry the mutated gene. If the 

 cell in which the mutation occurs is one of the two cells into 



