An Introduction to a Biology 



numbers of green-producing and yellow-producing gametes ; 

 he maintained that no fusion of characters (like a chemical 

 combination) takes place, but merely a mingling (like a 

 mechanical mixture). 



The proportions in which the D's, DR's, and R's occur 

 in the offspring of hybrids is certainly accounted for by this 

 theory. Let us (putting aside sex for brevity's sake) imagine 

 what happens when a hybrid is cross- or self -fertilised. 

 Imagine, first, a " green " germ-cell : it is an even chance 

 that it unites with a " green " or " yellow." 



a. Gx G 



b. Gx Y 



Imagine a " yellow " : it has an even chance of uniting with 

 a "green" or "yellow." 



c. YxG 



d. YxY 



Now b and c are the same ; so that the proportions in 

 which pure " greens," pure " yellows," and hybrids would be 

 produced, as the result of the random unions of the germ- 

 cells of two hybrids, would be IG : 2GY (hybrid) : 17, in 

 every four ; or, of course, 25% D : 50% DR : 25% R. And 

 this is exactly what happens as the result of actual breeding. 



(5) A DEVICE FOE EXPLAINING MENDEL'S THEORY 



I think this theory may be made clearer by a device 

 which has been useful to me ; some have imagined that it 

 is intended merely as an instance of the application of the 

 theory of probability ; this, however, is not the case ; its 

 value, such as it is, lies in the way the process is managed, 

 which has nothing to do with probability. 



All that is needed is some red and white counters. 



Mendel's conception of the gonad of a hybrid as an organ 

 containing germ-cells, 50% of which bear the dominant 

 character and 50% the recessive, can be easily imitated 

 by a bag containing equal numbers of red and white counters : 

 in fact, the production of the hybrid (or rather its gonad) 



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