320 AN INTRODUCTION TO MODERN GENETICS 



5. Variation 



The process on which animal or plant improvement immediately 

 depends is selection, but the results which can be obtained depend on 

 the sort of variations which are available. One of the main difficulties 

 of the breeder is to get into his material all the variants which he 

 desires; only then can he begin to select and combine together the 

 favourable factors. 



The variations on which useful breeds have been built up depend 

 mainly on gene mutations; chromosome changes, the other main 

 source of variation, usually lead to infertility and therefore are only 

 important in vegetatively propagated plants (but see p. 322). Not very 

 many favourable gene mutations affecting vigour, yield, etc., have 

 actually been observed at the time of their occurrence. An exception is 

 perhaps the mutation to a many-leaved, unbranched growth habit in 

 tobacco, which occurred in the Connecticut Cuban variety and may 

 prove to have economic importance.^ Somatic mutations may cause the 

 development of shoots with mutant characters, and these "bud- 

 mutations" have been extensively tested in vegetatively propagated 

 plants such as Citrus fruits, and several of commercial value have been 

 found both in Citrus fruits and apples. The nectarine is a bud mutation 

 from the hairy peach, to which it is recessive. 



The majority of spontaneous mutations lead to a decrease in vigour, 

 but they may still be valuable in horticultural plants if they produce 

 larger or more brightly coloured flowers, etc. The date of origin of the 

 factors affecting habit, flower colour and flower shape in plants such as 

 sweet-peas can be fairly accurately determined from old catalogues.^ 

 The enormous variety of such plants which are now available has been 

 formed by hybridization between the stocks in which the mutation 

 originally occurred, and a conscious application of Mendel's laws makes 

 it possible to produce any combination at will. 



The breeder seeking genes of a particular kind may succeed in 



finding them in the mixed progeny of commercial seed. We have 



already mentioned examples of the production of valuable new pure 



lines from such material. Another striking example is provided by the 



velvet bean Stizolobium, which was originally limited to the South- 



East United States until early maturing varieties, probably dependent 



on factor mutation, were found, which enabled it to be grown over a 



much wider area. 



1 Cf. Babcock and Clausen 1927. 



- Cf. Babcock and Clausen 1927, Crane and Lawrence 1934. 



