80 



The Source of Variability 



unequal crossing-over. When this process occurs at meiosis, one 

 chromosome of a dividing pair has a tandem duphcation of a gene, 

 and the other has a complete loss of that gene ( as is diagrammed 

 in Fig. 30). Thus no chemical change in the gene need have taken 



Original chromosomes ■> 



PS 



PS 



Crossover pattern 1 



Products -• 



Crossover pattern 2 



Products 



PIS 

 I 



ZZl 



-± 



P S 



Fig. 30. Diagram of the mechanism of unequal crossing-over and the re- 

 sulting tandem duplication of genie elements in some gametes and the loss of 

 genes in others. (Modified from Stadler. ) 



place. Stadler pointed out that unequal crossing-over of minute and 

 essentially undectectable areas could theoretically account for all 

 observed mutations. 



These conflicting ideas are not necessarily mutually exclusive, 

 but they do emphasize our lack of knowledge concerning both the 

 mechanism and causes of gene mutation. We know that mutations 

 arise and that the inheritance of those on the chromosomes is as- 

 sociated with a definite spot or condition of the chromosome. 

 Beyond that we are in the realm of speculation. 



ENVIRONMENT AND MUTATION 



To date, no evidence has been reported which indicates that the 

 normal environment produces genie mutations. Several instances 

 are on record, however, in which abnormal additives to the environ- 



