GENETICS IN MODERN BIOLOGY — BEADLE 409 



What happens to such typographical errors as are made ? First of 

 all, it is clear that the DNA molecules will replicate just as faithfully 

 whether the information in it makes sense or not. Its replication is 

 a purely mechanical one, it seems. Therefore mistakes in genetic 

 information will be perpetuated. 



It is obvious that if there were no way of eliminating errors in such 

 a process, such errors would accumulate from generation to genera- 

 tion. Perhaps an analogy will make this clear. If a typist types in 

 a purely mechanical way, never proofreading, never correcting, and 

 types successive copies of the same material always from the most 

 recently typed copy, she will accumulate mistakes at a rate depend- 

 ent on her accuracy until eventually the sense of the original mes- 

 sage will be entirely gone. In the same way this would have to happen 

 with genetic information if there were no way of taking care of 

 mistakes. With genetic information something does happen that 

 takes care of mistakes. By extending the analogy perhaps I can make 

 clear what does happen. The typist, typing mechanically, can cor- 

 rect a mistake by a second random typographical error, but obviously 

 the probability of this is extremely low. It is likewise so with genetic 

 information, and it is clear therefore that this is not the principal 

 way in which mistakes are prevented from accumulating. Let us pre- 

 tend the typist has an inspector standing beside her. '\'\nien she makes 

 a mistake, he says, "Throw that one away. Put it in the waste- 

 basket and start over." If in the next try she makes no mistake, he 

 says, "All right, now you may type another from the one you have 

 just finished." Each time she makes a perfect copy he allows her to 

 go ahead, but each time she makes a mistake he insists she throw the 

 copy away. That is what happens with genetic information. The 

 inspector is analogous to natural selection. Bad sets of specifications 

 in man are eliminated by natural selection. 



A more dramatic term for elimination of unfavorable specifications 

 by natural selection is "genetic death," as used by Dr. H. J. Muller. 

 Individuals developed from unfavorable specifications do not repro- 

 duce at the normal rate, and ultimately a line so handicapped dies 

 out. To avoid progressive accumulation of mistakes from generation 

 to generation, it is obvious that every error in replication that is 

 unfavorable must be compensated for by the equivalent of a genetic 

 death. That is why geneticists are concerned about factors that in- 

 crease the mutation rates. 



You may quite properly ask, "Are there no favorable mutations?" 

 The answer is yes, there are occasional favorable mutations ; they are, 

 in fact, the basis of organic evolution. 



However, because many mutations involve subtle changes that may 

 be favorable under special circumstances of environment or over-all 



