48 GENETIC VARIATIONS 



In view of the fact that mutations may remain long hidden, 

 it is fortunate for purposes of study of the occurrence of muta- 

 tions that in many organisms a certain number of genes in 

 one of the chromosomes, known as the X-chromosome, are in 

 one of the two sexes not paired, but single. In most organisms 

 that have been extensively studied from this point of view, it 

 is the male that has a certain number of genes that are single, 

 in the X-chromosome. If any of these unpaired genes become 

 mutated, in the fertilized egg or in the cells from which the 

 ovum is derived, the effect of the mutation appears at once, in 

 case the individual developed from that egg is a male. Thus 

 the time and place of the occurrence of gene mutations has 

 been studied most extensively in the genes of this X-chromo- 

 some. In recent years, however, an elaborate and adequate 

 technique has been devised for bringing to light almost at 

 once the effect of mutations in the genes of any of the 

 chromosomes. In this way much has been learned as to the 

 time and place of the occurrence of mutations, 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 organism. 

 The mutation occurs in but one single cell. If this is the single 

 cell of the fertilized egg, then all the cells of the body devel- 

 oped from that egg will carry the mutated gene. If the cell in 

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

 the fertilized egg divides, then only half the body will carry 

 that mutation. If the mutation occurs in a cell at a still later 

 stage of development, only a small part of the body will show 

 the mutation. Many cases of such kinds have been thoroughly 

 studied, cases in which one part of the body shows the muta- 

 tion, the rest not. The general upshot of all these studies is to 

 show, as remarked before, that gene mutations may occur in 

 any cell of the body, at any period in life. 



