MENDELISM 229 



recessives which always bred true. Having determined 

 a general scheme of inheritance, which experiment 

 showed to hold good for each of the seven pairs of alter- 

 native characters with which he worked, Mendel set 

 himself to providing a theoretical interpretation of this 

 scheme which, as he clearly reahsed, must be in terms 

 of germ cells. He conceived of the gametes as bearers 

 of something capable of giving rise to the characters of 

 the plant, but he regarded any individual gamete as being 

 able to carry one and one only of an alternative pair of 

 characters. A given gamete could carry tallness or 

 dwarfness, but not both. The two were mutually ex- 

 clusive so far as the gamete was concerned. It must 

 be pure for one or the other of such a pair, and this 

 conception of the purity of the gametes is the most 

 essential part of Mendel's theory." 



Let me now attempt to show you how far Mendel's 

 theoretical explanation was in harmony with his experi- 

 mental results, and in doing so I shall follow, but at the 

 same time condense, the admirable sketch of the whole 

 subject by the author from whom I have just quoted. 



Since they bred true, the original tall and dwarf 

 parents gave off respectively gametes with tall and with 

 dwarf characters. When these gametes fused the zygotes 

 naturally carried both characters ; but since tallness is 

 dominant the new plants will all be tall, although half 

 the gametes produced by any one of them will have the 

 tall character and the other half the dwarf. Let us 

 suppose that the plant has 100 ovules, 50 with the tall 

 character and 50 with the dwarf. Similarly, if it has 

 100 pollen grains, 50 will have the tall and 50 the dwarf 

 character. Each ovule with the tall character has an 

 equal chance of being fertihsed by a tall or a dwarf 

 pollen grain, and each ovule with the dwarf character 

 has similarly an equal chance of being fertihsed by a 

 dwarf or a tall pollen grain. Thus on an average 25 tall 

 ovules will be fertihsed by 25 tall pollen grains, and 



