SOME PRINCIPLES OF PLANT BREEDING 477 



hybridize according to Mendel's law, nothing would be gained 

 because only two forms are obtained, which are exactly like the 

 parents (paragraph 646). These are called monohybrids. In 

 crossing varieties which differ in respect to two characters which 

 behave according to Mendel's law, dihybrids are produced and 

 four combinations are possible, resulting in successive genera- 

 tions in four different varieties which breed true, two of them 

 like the parents and two new varieties. For example, in crossing 

 the blue-flowered thorn apple, or jimson weed, with the white- 

 flowered smooth one, besides the two parent forms, two new ones 

 are obtained, a blue-flowered smooth one and a white-flowered 

 thorny one. So in the case of three different unit characters in 

 the parents eight combinations are possible, two of them like 

 the parent and six new ones. In these cases it is possible to 

 predict in advance the new forms which can be obtained and 

 selection can be made from them. 



Very important results have in fact been obtained recently by 

 Biffen of Cambridge University in England. The wheat com- 

 monly grown in England is a soft wheat, i.e., with a high per- 

 centage of starch and low protein content, but it also produces a 

 large yield. He worked to develop a variety of wheat having a 

 high protein content, with the high productivity of the English 

 wheat, and also which would be free from the disease known as 

 rust. He found by experimental work that these and some other 

 characters behaved as unit characters in Mendelian fashion (par- 

 agraph 646), as follows: 



Having cropping quality dominant to poor cropping quality. 

 Glutinous wheat (high per cent protein) dominant to starchy wheat. 

 Rusty plants 'dominant to resistant plants. 

 Early wheat dominant to late. 

 Red grain dominant to white. 

 Rough chaff dominant to smooth. 

 Bearded wheat dominant to bald. 

 Stiff stems dominant to weak. 



By crossing in various combinations he was able to produce 

 a variety of wheat possessing the heavy cropping qualities and 



