DOMINANTS AND RECESSIVES 153 



254. Mendel supposed that the 'tall 'and 'dwarf characters 

 were represented by units in the substance we now call germ- 

 plasm. He thought that each pollen grain and unfertilized ovule 

 (gamete, or germ-cell) contained one unit for stature, and therefore 

 that each fertilized ovule (zygote) contained two. 1 One only of 

 these units directed development, and the character which resulted 

 from its influence he termed the * dominant.' The latent character 

 was termed the ' recessive.' 



255. He supposed further that before the development of the 

 germ-cells of the mongrel plant which sprang from the zygote, the 

 units multiplied and separated, so that in each of its pollen grains 

 and ovules only one unit was present, a dominant unit or a reces- 

 sive unit. When the plant flowered and self-fertilization again 

 took place, it was a matter of chance whether these units paired 

 with their own kind in the zygote, or with units of the opposite 

 type. If a dominant unit met another dominant the result was a 

 'pure ' dominant plant, which never had, nor could have a recessive 

 descendant so long as self-fertilization continued. In like manner, 

 if a recessive unit met another recessive the line became exclusively 

 recessive. But, if a dominant met a recessive, the resulting plant 

 was an l impure* dominant, which looked like the pure dominant, but 

 was really different in that it was a hybrid having descendants of 

 the three types, pure dominants, recessives, and impure dominants. 

 Every plant which displayed the recessive character was pure of 

 course, for otherwise the dominant element would have been patent. 



256. If the reader will shake together in a bag a hundred 

 black and a hundred white buttons, representing dominant and 

 recessive units respectively, and then withdraw them at hazard in 

 pairs, he will find that, on the average, half the buttons will be 

 paired with their own kind and half with the opposite kind. He 

 will then have approximately twenty-five black pairs representing 

 pure dominants, twenty - five white pairs representing recessives, 

 and fifty black-and-white pairs representing impure dominants. 

 The proportion of pure dominants, recessives, and impure domi- 

 nants, therefore, is supposed to be regulated by the law of chance. 



257. It was found that peas possessed several Mendelian 

 characters besides length of stem, for example colour of cotyle- 

 dons (yellow or green), colour of seed skins (brownish or white), 

 colour of flowers (purple or white), shape of ripe pods( inflated or 

 constricted), colour of unripe pods (green or yellow), position of 

 flowers (axial or terminal), and shape of seeds (round or wrinkled). 



1 See Bateson, Mendel's Principles of Heredity, pp. 338-9. 



