372 GENETICS IN RELATION TO AGRICULTURE 



After correcting the yield of individual plots for differences in soil fer- 

 tility (see Chapter XXV) it was found that the 12 pure lines averaged 

 to yield 80.8 bushels per acre against 75.2 bushels for the 11 commercial 

 varieties. "Only 4 of the commercial varieties gave a better yield 

 than the poorest of the pure lines. In all cases the average yield of the 

 pure lines selected from a given variety exceeded the yield of the parent 

 variety." Of the 18 commercial varieties with which they started only 

 3 are represented among the 12 pure lines. It was found that these pure 

 lines closely resemble their respective parents in morphological characters 

 and concluded, therefore, that mutations in the physiological characters 

 which result in higher yield are not necessarily associated with changes 

 in morphological characters of the plant or grain. 



Even more striking results in some respects have been obtained by 

 Clark by means of head-to-row selections of Ghirka spring wheat in North 

 Dakota. Starting with 300 individual plant selections in 1909, in spite 

 of the destruction of all the cultures by hail in 1912, after 5 years' 

 work two pure lines were found, one of which was superior to unselected 

 Ghirka in all characters except crude protein content and the other in all 

 characters except volume of the baked loaf. 



In all such work the importance of beginning with large numbers must 

 be emphasized. Other things being equal as the number of selected in- 

 dividuals increases the chances of locating the desired variants increase. 

 The same holds true when attempting to locate aberrant individuals by 

 inspection of young seedlings, nursery stock, etc. 



Propagation of Mutations. The question of how to preserve and prop- 

 pagate a desirable mutation becomes a problem only in plants which are 

 normally cross-fertilized. In self-fertilized species the new form exists 

 as a pure line and need only be isolated. Similarly in plants that are 

 propagated vegetatively usually there is no difficulty in multiplying a 

 new variet} 7 . This method has been applied to such crop plants as 

 alfalfa with great success. But in a cross-fertilized plant in which 

 vegetative propagation is impracticable the method of procedure will 

 depend upon circumstances. If in a given species the plants are self- 

 fertile the appearance of only a single plant of a new form makes it possible 

 to test its genetic constitution and if it is a mutation to multiply it. If 

 it is heterozygous for the factor conditioning the new character or char- 

 acters it will of course be necessary to select the best individuals from 

 the next generation. This situation will confront the breeder only in the 

 case of mutant factors which are dominant or partially dominant when in 

 the heterozygous condition. When the mutant factor shows partial 

 dominance in a heterozygote but segregates as a Mendelian recessive 

 there will be no difficulty in establishing a pure strain, but should it 

 segregate as a Mendelian dominant it becomes necessary to test a number 



