Genes and Mutations 55 



added contains a gene, and the conclusion is inevitable that in these 

 instances the gene is in fact a nucleic acid (dna). 



How the different types of one kind of bacteria arise in the first in- 

 stance is not very clear. But it is known that if colonies of bacteria 

 are kept, occasionally a change will occur in a bacterium which is 

 transmitted to its progeny. Such a change, known as a mutation, is 

 not a very infrequent phenomenon and is responsible for the many 

 strains of bacteria which exist. 



Mutations of this kind are probably responsible for the resistance 

 to drugs which bacteria develop. For example, it is well known that 

 common bacteria present in infections may develop a resistance to 

 penicillin. This means that a change has occurred, with the develop- 

 ment of a new strain which can multiply in the presence of penicillin, 

 because it has a somewhat different constitution. 



Hotchkiss, in 1951, made the surprising discovery that penicillin- 

 sensitive strains of Pneumococcus can be transformed into penicillin- 

 resistant strains simply by adding to the former dna derived from 

 the latter. In other words the property of being able to grow and mul- 

 tiply in the presence of penicillin is conferred by addition of dna, and 

 again, once acquired, this ability is transmitted to descendants. The 

 same phenomenon has been found in other cases, e.g. the streptomy- 

 cin-resistance which is developed by a certain type of influenza 

 bacillus. 



All these experiments supported the idea that dna alone was 

 capable of behaving as a gene. This demonstration in higher organ- 

 isms is, of course, much more difficult because there is no way in 

 which we can introduce extraneous dna into the germ cells of ani- 

 mals. It would certainly be fascinating if it were possible to remove 

 one chromosome and replace it by another with a different hereditary 

 background, or if it were possible to change the characteristics of an 

 animal by introducing dna from another species. The difficulty is to 

 get the dna into appropriate cells and as we shall see later animals re- 

 ject nucleoproteins even from other individuals of their own kind. If 

 this were not so it might be possible to make all sorts of composite 

 animals. It has been reported recently by J. Benoit, P. Leroy and C. 

 and R. Vendrely that the injection of dna made from ducks of one 

 variety (Khaki Campbells) into ducks of a second variety (Pekin) 

 from the eighth day after hatching, caused definite changes in the 

 latter. Usually the Khaki Campbells are large birds with yellow 

 beaks and creamy-yellow plumage, while the Pekins are smaller 

 ducks with greenish-black beaks. The beaks of the treated ducks de- 

 veloped bold black smudges against a background which was either 

 yellow or rose pink. It was suggested that these modifications 



