CHAPTER V 



INHERITANCE IN PURE LINES 



THE term " pure line " is a new one in the science of 

 heredity. It was first used by Professor Johannsen in 

 his important paper, " On Inheritance in Populations 

 and in Pure Lines," published in 1903. 



A pure line may be described as including all the 

 descendants of a single individual, belonging to a race 

 which is reproduced exclusively by self-fertilisation. 

 This implies that the germ plasm of any individual has 

 only one origin. Descent in each generation is from 

 one parent only, hence there is no opportunity for the 

 mixing of different germ plasms or for the recombina- 

 tion of characters derived from different parents. 



Inheritance in pure lines is therefore the simplest 

 possible case, and it will be well to consider it 

 before we attempt to study those which are more 

 complex. 



Johannsen's original experiments were carried out 

 with beans and barley. The characters of the former 

 which he studied were the w r eight and the relative 

 breadth of the seed, of the latter the tendency to 

 blindness, or the occurrence of barren flowers in the 

 head. 



In a random sample of beans, which had been grown 

 from nineteen different original parents, the size was 

 found to show ordinary variability. The curve of 

 variability was very nearly normal, and there seemed to 

 be every probability that all the beans of the " popula- 

 tion " belonged to a single type. The average weight 

 was 478*9 milligrams, and the standard deviation 

 was 95-3. 



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