CHAPTER XVI 

 HEREDITY IN PURE LINES 



THE NATURE OF PURE LINES 



When all the individuals in a family line — offspring, parents, grand- 

 parents, and so forth — have just the same hereditary materials (genes 

 and chromosomes), the whole family connection is known as a "pure 

 line." Pure lines may originate in at least three ways : (a) by asexual 

 modes of reproduction, such as fission and budding; (b) by sexual re- 

 production of hermaphrodite or monoecious parents, such a parent 

 being a double-sexed, male-female, individual, in which both eggs and 

 sperms are provided by the same parent; (c) by prolonged close in- 

 breeding, brother-and-sister mating, for many generations. 



Members of pure lines, if they are genuinely pure, will all be exactly 

 alike in their bodily characters if they develop under identical environ- 

 mental conditions, unless some change, such as a mutation, occurs. 

 When differences occur among members of a pure line, we can be sure 

 that such differences are not innate, but are due to external, or en- 

 vironmental, causes. 



Much may be learned about heredity from a study of pedigrees of 

 individuals in pure lines. In pure lines heredity presents itself in its 

 simplest form, uncomplicated by the mechanism of diversity. Here 

 we can study the operations of heredity modified by only one other 

 factor, the environment. Some of the facts that are brought to light 

 by our studies of pure-line heredity will serve to simplify and elucidate 

 the more complex aspects of Mendelian heredity. 



THE PURE-LINE EXPERIMENTS OE JOHANNSEN 



One familiar with the field of genetics never thinks of pure-line 

 breeding without thinking of Johannsen and his famous nineteen beans. 

 He was interested in pure -line breeding primarily as a means of im- 

 proving certain kinds of beans for economic reasons. He thought he 

 might be able to produce uniformly bigger beans by always planting 

 only the biggest seeds. In order to avoid the complexities of mixed 

 races, he worked with pure lines, keeping each pure line quite separate 

 from the others. Although he experimented with nineteen pure lines, 

 we shall not attempt to follow up the results of more than two of them. 



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