THE PRACTICAL VALUE OF GENETICS 343 



The diagrams of Mcndolian hybrids (Fig. 159, 161, 163) show 

 that any combination, no matter how many unit characters are 

 involved, appears in a hmited number of individuals in each genera- 

 tion after the first fihal in the homozygous state. When once 

 secured, individuals of this kind bearing a new combination of 

 characters are the potential heads of a new race. 



If, for example, one of our American roasting-ear enthusiasts 

 were cast on an uninhal:)itcd island with only yellow field corn and 

 white sweet corn in his possession he could secure at l(^ast an 

 imitation of his beloved yellow bantam sweet corn in the second 

 generation. By cross-fertilizing the two strains he would secure 

 a yellow-starchy hybrid of the zygotic formula YySs. When 

 these seeds were planted and the new generation carefully inbred, 

 four kinds of grains would appear, viz., yellow starchy, yellow 

 sweet, white starchy and white sweet. Among the yellow sweet 

 grains would be two thirds Yyss and one third YYss. The latter 

 could produce nothing but yellow sweet corn. 



The sources of valuable characters are those already mentioned. 

 The range of fortuitous variation supplies some, as in the case 

 of corn which varies in starch, sugar and oil content. Mutations 

 useful to man have also appeared. Babcock and Clausen record 

 the following mutants of cultivated plants: 



Early maturing varieties of the Florida Velvet Bean. 



Tobacco, including one mutant of Connecticut Cuban which 

 showed an increase of 90 per cent in yield. 



Sugar beets with increased sugar content. 



Various useful mutations of cotton, hemp, rye and sunflower. 



Plant Hybrids. Such hybrids are valuable for new combina- 

 tions and for increased vigor. Hybrids of different strains of 

 corn are commonly reported to show a marked increase in yield 

 over either of their parents, sometimes amounting to 250 per cent 

 (Fig. 192). Crosses have been made of many different varieties 

 of corn, and in most cases this increase in yield has been observed. 

 Such increase of vigor in hybrids, whether plants or animals, has 

 been called heterosis by Shull. The following table gives the 

 results of some of the crosses of corn. 



