316 



AN INTRODUCTION TO MODERN GENETICS 



in this way, particularly in wheat. ^ The variety Red Fife, which spread 

 through the whole wheat-growing area of Canada in the latter part of 

 the last century, originated from a single plant. A farmer named Fife 

 obtained seed, which originally came from Russia, and planted it in the 



W E ^J G H T /\ 



X A 



Fig. 131. Pure Lines. — If one plots the frequency of different weights of, e.g. 

 beans, in a large population, a curve is obtained like that marked A. This is the 

 sum of the frequency curves of the different individual families of beans. Since 

 beans are self-fertilizing, any heterozygote (6b) v^ill have half its offspring homo- 

 zygous (66 or bb), and thus after some generations nearly all the individuals 

 in a self-fertilizing population become homozygous. All the members of a family 

 are therefore genetically identical, and the variation in weight within a family is 

 due only to variations in environmental conditions. The lower part of the figure 

 shows that if we take the lightest and the heaviest members of a family (e.g. that 

 giving the dashed frequency curve) their progeny will, under the same con- 

 ditions, give the same frequency curve, which proves that there is no genetic 

 difference between them. The same is true for any other family, e.g. the dotted one. 

 Each family is a pure line. 



Spring only to find that it was a winter sort. Only one plant ripened, 

 and from this plant the whole Red Fife variety was derived. Another 

 and more recent example is the variety Kanred, produced by the 

 Kansas experiment station as the best of 554 pure lines started in 1908 

 from single plants of the Russian variety Crimean. The preliminary 

 testing took eight years, but in the seven years after being put on the 

 market Kanred proved so successful that it was grown on approxi- 

 mately two million acres. 



* Rev. Hunter and Leake 1933. 



