126 HOW ANIMALS DEVELOP 



lography of certain complicated organic compounds. 

 A third has been the use of such modern technical 

 devices as the electron microscope, the ultra-centrifuge, 

 and radioactive isotopes. The whole story is a very 

 good example of the way in which different parts of 

 science can come together and reinforce one another. 

 When two similar chromosomes are paired together 

 in a primordial germ cell, as we described on page 30, 

 they sometimes break at corresponding places and the 

 two pieces rejoin in the wrong order, so that what had 

 been the end of chromosome 2 has now become joined 

 to the beginning of chromosome 1, and vice versa. A 

 study of this process, which is known as recombina- 

 tion, has always been one of the most powerful tools 

 in genetics. It is easy to see that if two genes lie on a 

 chromosome and are quite a long distance apart, 

 breakage and rejoining is quite likely to occur between 

 them, whereas if they lie very close together the chance 

 of this happening in that particular stretch of chromo- 

 some is much less. We can therefore discover how far 

 apart two genes are by determining how often the 

 recombination happens between them. One of the 

 most important pieces of recent evidence about the 

 nature of genes came to light when it became possible 

 to study recombinations which happen excessively 

 rarely. Bacteria and viruses are such small organisms 

 that they can be cultivated in millions or even billions. 

 A study of their genetics, of course, involves many 

 highly elaborate techniques, but when these were 

 eventually worked out it became possible to look for 



