MOSAIC CHARACTER OF OFFSPRING OF HYBRIDS 241 



and other plant breeders in their practice of combining by hy- 

 bridization the various good qualities of different varieties while 

 eliminating those that are undesirable. It must be remembered, 

 however, that they arrive at their results only after searching 

 over hundreds, and even thousands, of the offspring of hybrids. 



We may refer to Fig. 134 to illustrate another fact of impor- 

 tance. K, L, M, and N are, as stated, the different possible 

 offspring of D 4 under self-fertilization. Now how can we deter- 

 mine the degree of purity of these varieties ? We can, of course, 

 test their purity by the character of their offspring produced by 

 self-fertilization, and this would be an adequate and final test; 

 but there is another way of arriving at a partial answer to our 

 question. We know that when the offspring of a hybrid shows 

 recessive characters it is pure to those characters, for if the con- 

 trasting dominant were present it would be expressed and the 

 recessive would not appear. The offspring of K, for instance, 

 produced by self-fertilization, would all have smooth stems 

 and terminal flowers, but they might vary in color of flower and 

 form of leaf. The offspring of L would all have terminal flowers, 

 smooth stems, and long leaflets, but might vary in color of flower. 

 M would always give terminal blue flowers and smooth stems, 

 while possibly varying in form of leaflets. N would have none 

 but pure offspring, since, being recessive in all of its characters, 

 it must be homozygote to all of them. 



Mosaic Character of Offspring of Hybrids. It would 

 appear from all that has now been said that the offspring of a 

 hybrid are mosaics of the characters of the original parents of the 

 hybrid, and that different combinations of these characters may 

 appear in the different individuals. Since in spore production 

 the characters may separate and recombine independently, like 

 the blocks with which children build different forms of houses, 

 etc., according to their fancy, or like the bits of glass in a kaleido- 

 scope which fall apart and reassemble in various patterns, they 

 have been called unit characters. We may conceive of a unit 

 character as the expression of the hereditary power of a pangene; 

 or we may think of it as this power itself. 



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