56 Inside the Living Cell 



were also transmitted to the progeny; but further experiments are 

 required before it can be regarded as certain that dna injected into 

 animals can really change the genetic characteristics. 



The biological method of doing this experiment is to mate male 

 and female organisms of the same species which have a different 

 genetic constitution. As I have mentioned above, 'crossing over' of 

 material derived from the two parents occurs and the progeny will 

 carry different combinations of the genes available. From a study of 

 these, it is possible to distinguish the genes of different individuals. 



An enormous amount of work of this kind has been done with the 

 fruit fly, or drosophila, which has been bred through many genera- 

 tions. These flies are very convenient. A new generation appears in 

 about a week and they are simple to keep and to feed. If a large num- 

 ber of flies is studied, every now and then a spontaneous change or 

 mutation occurs, which is transmitted to the progeny of this indivi- 

 dual in accordance with the laws of genetics. In the course of many 

 years of work by Morgan and his colleagues in Columbia University, 

 no less than 800 distinct mutations were observed, which could be 

 bred true to type and were undoubtedly due to distinct changes in the 

 genes. The chromosomes of the flies were examined at the same time. 

 This is easy to do because it happens that, in the larva state, the 

 chromosomes of their salivary glands are enormously developed and 

 can be seen even with a low power microscope (Plate 4) to have a 

 banded structure. Changes in the chromosome corresponding to the 

 different mutations can easily be seen, so that it is possible to identify 

 a mutation with a particular part of the chromosome. In fact chromo- 

 some maps showing the positions of many genes have been worked 

 out. 



The essential characteristic of a gene is that it has a permanence 

 which enables the same character to be transmitted through innumer- 

 able generations. But at the same time new characters sometimes 

 appear (or old characters disappear). It has been estimated that in 

 the fruit fly, on the average, an individual gene may undergo a muta- 

 tion in something like a million generations. This means that no mis- 

 take is made in the copying of the original gene until on the average 

 correct reproduction has occurred a million times— which would 

 occupy, with the fruit fly, between a thousand and ten thousand years. 

 Professor J. B. S. Haidane has calculated that with human genes the 

 likelihood of a mutation may be only of the order of once in a mil- 

 lion years. Nevertheless the fact that such changes in the genie 

 material can occur is of great importance because without it evolu- 

 tion could not take place. The majority of gene changes will be dis- 

 advantageous and will cause the death of the organism or their non- 



