GENETICS 685 



tion unit being much smaller and the functional unit perhaps larger 

 than the unit of crossing over. Our concept of the intimate nature of 

 the gene is being revised constantly as new experimental evidence ap- 

 pears. 



288. Changes in Genes: Mutations 



Although genes are remarkably stable and are transmitted to suc- 

 ceeding generations with gieat fidelity, they do, from time to time, 

 undergo changes, called mutations. After a gene has mutated to a new 

 form, this new form is stable and usually has no greater tendency to 

 change again than the original gene. 



Two types of mutation are distinguished. Some, called chromo- 

 somal mutations, are accompanied by some visible change in the struc- 

 ture of the chromosome— the deletion or duplication of a small segment 

 of the chromosome, the translocation of a segment of chromosome to a 

 new position in a different chromosome, or the inversion, turning end 

 for end, of a segment of chromosome. Others, called point mutations, 

 have no visible change in chromosome structure and we assume that 

 these involve such small alterations at the molecular level that they are 

 not visible. From our current theory that genes are complex nucleic 

 acid molecules, we can guess that mutations involve some change in the 

 order or arrangement of the nucleotide units of the DNA. 



Gene mutations can be induced by exposing the cell to radiation; 

 x-rays, gamma rays, cosmic rays, ultraviolet rays and all the types of 

 radiation which are by-products of atomic power are effective mutation 

 agents. Mutations do occur spontaneously at low but measurable rates 

 which are characteristic of the species and of the gene; some genes are 

 much more "mutable" than others. Natural radiations such as cosmic 

 rays probably play some role in causing spontaneous mutations, but 

 there are undoubtedly other important factors. The rates of spontaneous 

 mutation of different human genes range from 1 X l^-^ to 1 X 10~^ 

 mutations per gene per generation. Since man has a total of some 2.5 X 

 10^ genes, this means that the total mutation rate is on the order of one 

 mutation per person per generation. Each one of us, in other words, has 

 some mutant gene that neither of our parents had. 



289. Gene Action 



There is a tremendous amplification of effect in the train of events 

 from single pairs of ionizations, produced by the passage of x-rays 

 through a tissue, to a gene mutation which in turn produces the altered 

 phenotypic expression. To explain this, genes are believed to act as 

 catalysts for the production of enzymes. Enzymes are believed to owe 

 their specificity to the specific configuration of the surface of the molecule 

 (p. 69). Only those substances whose molecules have the proper shape 

 can fit on the surface of the enzyme, make contact at a number of points, 

 and form an enzyme-substrate complex. According to our present theory, 

 the surface of the gene has a comparable specific conformation, and this 



