372 GENETICS IN RELATION TO AGRICULTURE 
After correcting the yield of individual plots for differences in soil fer- 
tility (see Chapter X XV) 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. 
Tn 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, ete. 
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-fertilhzed. 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 variety. 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 1t 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 
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