310 AN INTRODUCTION TO MODERN GENETICS 



view, the production of satisfactory animals and plants becomes a 

 question of the production of satisfactory genotypes. Since no way 

 exists at present of manufacturing particular genes to order, the prob- 

 lem reduces to that of selecting and bringing together the genes which 

 chance has offered. The methods which are relied on for this purpose 

 are selection, crossing, and inbreeding. These methods have probably 

 been used to some extent ever since plants and animals were first 

 domesticated, but we now understand more clearly what are their 

 particular uses. By selection, we aim to breed from an organism con- 

 taining useful genes, which are thus perpetuated. Crossing has several 

 functions. By permitting segregation in the F2 and subsequent genera- 

 tions it allows us to break up a genotype into its constituent genes, and 

 thus to select those which we require and rejea the others. At the same 

 time, it enables us to combine in a single individual the genes which 

 previously had existed in different lines. Finally, by inbreeding, we 

 perpetuate useful lines already in existence, and, combining inbreeding 

 with selection, we can purify them by reducing them to a homozygous 

 condition. 



2. Selection of one Parent only 



The most elementary form of selection practised by breeders is the 

 selection of one parent only, the choice of the other being left to chance. 

 This so-called mass selection has probably been practised since the 

 earliest times; a careful farmer used for planting the seed taken from 

 the healthiest and most productive of his plants. If a plant is self- 

 fertilized, selection of one parent is in effect selection of both, and it 

 would be possible to obtain a uniform and valuable variety quite 

 rapidly in this way. In a cross-fertilized organism, in which only half 

 the genetic factors of the offspring are contributed by the selected 

 parent, one can only expect a slow accumulation of beneficial genes. In 

 practice, the steady improvement may continue over very long periods, 

 since the characters selected for, such as yield, are often such as to be 

 the resultants of complex physiological processes and are therefore 

 likely to be affeaed by many factors, which take a long time to collect. 

 In a well-known case,^ maize was selected for high and low protein 

 content and high and low oil content; after more than twenty years 

 each of the four selected lines was still changing, though slowly, in the 

 required direction. 



This laborious method of improvement has been of the greatest 



^ Winter 1929. 



