An Introduction to a Biology 



dicated by the numbers prefixed to them — 25 g'g' (or g') 

 25 g'g, 25 gg\ and 25 gg, or {g'g and gg' being the same) 25 g\ 

 50 gr'gr, and 25 g — which, the reader will remember, is the 

 actual result. 



This proportion is simply the result of the random union 

 of the gametes of the hybrids, and can be illustrated by 

 making pairs of counters by taking one of the pair at random 

 from a hat containing red (R) and white (W) counters in 

 equal numbers, and the other of them from another hat 

 with similar contents. The result of a large number of 

 trials \\dll be in percentage 25 RR, 25 RW, 25 WR, and 

 25 WW, or 25 RR, 50 RW, 25 WW. 



The difference between the above-outlined Mendelian and 

 Weismannian theories is that while the former tries to account 

 for the segregation and not for the reversion, the latter tries 

 to account for the reversion and not for the segregation. It 

 is when we fix our attention on that part of the MendeHan 

 theory which refers to the nature of the gametes of the 

 hybrid that we see what the doctrine of gametic purity really 

 means, how profoundly new and definite a thing that theory 

 is, and how mdely it dift'ers from any other theory of heredity 

 whatsoever. 



Let us imagine that we have one of our hybrids, with 

 its rich brown coat and black bead-Hke eyes, before us ; a 

 mouse that we might easily mistake for a wild one caught 

 in a trap, if we did not know its parentage. According to 

 the particular Mendelian theory we have been discussing, 

 none of the gametes of this mouse contain an element representing 

 the character of the animal which hears it — namely, g'g. But 

 half of the gametes bear the element, g' and half g. The 

 fact that 50 % of the children of such hybrids are hke their 

 parents is not due to the presence in the germ-cells of their 

 parents of any elements representing their own characters, 

 but to the chance union of g' and g, borne by different parents. 

 A hybrid in this generation — F^ — is, therefore, never produced 

 by the union of similar, but always by the union of dissimilar, 



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