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ZOOLOGY 



pairs, A-a, B-b, C-c, etc., then A is just as likely to 

 occur with B as with b, and B with C as with c. 



Modern research has shown that while the simple 

 cases recorded by Mendel are typical, there are nu- 

 merous exceptions, which are explained by various ex- 

 tensions of the theory, without at all contradicting 

 Mendel's essential results. 



8. Not only are the determiners inherited as units, 

 but they ordinarily remain unmodified, whether they 

 produce any visible features or not. We are reminded 

 of the phenomena of chemistry. Thus oxygen and 

 hydrogen, two gases, when united become water, which 

 is not at all like either of them. An atom of oxygen 

 may today be part of water, tomorrow part of iron 

 rust, and the third day again appear as oxygen, not in 

 the least changed by the temporary loss of its ordinary 

 properties. It is quite certain that the determiners are 

 not chemical atoms, they are doubtless thousands of 

 times more complex than that; nor do they form 

 chemical combinations as do the atoms, but they re- 

 semble them in their stability and reappearance after 

 having seemed to cease to exist. 



9. Having thus postulated the existence of inde- 

 pendently inherited determiners, pairs of which are 

 mutually exclusive, we can proceed to develop a theory 

 of Mendelian inheritance. Each plant (or animal) is 

 double or duplex, in the sense that it inherits or receives 

 one set of determiners from each parent. Should the 

 two sets be alike, we say that the individual is pure- 

 bred, technically homozygous; should they be different, 

 it is cross-bred, or heterozygous. We may express the 

 facts by formulae, as did Mendel, in which, however, we 

 cite only the characters with which we are immediately 

 concerned. Let a pure-bred tall pea (TT) be crossed 



