138 Outline of Genetics 



as they were inbred. The other two are exactly Hke the original 

 Fi hybrid and therefore semi-sterile, having one-half sterile gameto- 

 phytes. The whole dynasty may be represented as follows: 



This is a very ingenious scheme, and, like most others, should 

 be tested by further experiments. To a certain extent it has already 

 met this test, for Belling (3) has subsequently reported a few 

 more generations in which the breeding results were entirely con- 

 sistent with those of the earlier generations. Also he has dis- 

 covered two new races of beans which give similar results. 



C. Self-sterihty. 



This is a phenomenon which has loomed up as a great practical 

 problem during the last few years. Where pollen and ovules are 

 entirely healthy, and functional in out crosses, but quite ineffective 

 in bringing about self-fertilization, the condition of self-sterility 

 is said to exist. This frequently appears in certain of our impor- 

 tant horticultural plants, such as apples and plums, so that it has 

 often been found necessary to include a mixture of several varieties 

 within the orchard in order to insure effective pollination. IVIany 

 investigators have been attacking this problem with various tech- 

 niques, the most critical work from the point of view of geneticists 

 being that of East on Nicotiana. 



By crossing self-sterile with self-fertile species, East (6) has 

 demonstrated that self-sterility behaves in inheritance like a 

 simple IVIendelian recessive to self-fertility; a single gene evidently 

 determines the difference between the two conditions. East (4) 

 has further made an attempt to analyze the relations of self- 



