INHERITANCE— 1MPR0 VEMENT ?! ] ' BREEDING 2 1 1 



true to type as possible. The usual result of mating like with CHAP, 

 like, is to produce like, provided always that the strains are V ' 

 pure ; the last point is essential. There are exceptions to this 

 rule, due to the interaction of other factors (IF 131) which we 

 need not discuss at the moment as they do not affect the 

 principle ; we refer to such cases as the crossing of two 

 white-grained breeds of maize, which sometimes results in the 

 production of purple grain ; and the crossing of two dwarfs, 

 which in some instances results in the production of tall plants. 



It is equally important to remember that by crossing the 

 unlike we usually produce unlike, at least in the second 

 generation. 



Selection can carry us to a certain point, but no farther ; it 

 isolates characters and eliminates the unfit, but it cannot add 

 characters which are not there. This is accomplished by cross- 

 breeding. "Any permanent improvement that is made by 

 selection is merely the separation of one of the extreme 

 biotypes. When an extreme line is entirely separated, however, 

 selection of its extreme fluctuations causes no change (or at 

 least no permanent change) of type, because there is almost 

 complete regression to the mode of its line" {East, 2). 



166. Effect of Inbreeding. — Strict inbreeding gradually leads 

 to the isolation of the homozygous type, as was pointed out by 

 Mendel, and as is demonstrated in the following diagram, for 

 which the writer is indebted to Professor Punnett : — 



OR 



(Heterozygous Parent, inbred) 



I 



I 

 F 1 4 DD selfed 



F., 16 DD 



F., 64 DD 



8 DD 



DD 



I 



I 

 SDR selfed 



I 

 16 DR 



I 



S RR 



I 

 4 RR selfed 



16 RR 



16 DD 32 DR 16 RR 32 RR 64 RR 



This gives, in the F 3 generation, 112 DD (pure extracted 

 dominants), 112 RR (pure extracted recessives), and only 32 

 DR (heterozygotes). This scheme supposes that each plant 

 of the progeny produces but four offspring and that chances 



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