136 PLANT GROWTH 



dom. If two genes are close together on the chromosome, 

 the chances of a crossing over occurring between them is less 

 than if the genes were farther apart. Thus, if two genes are 

 close together the recombination classes are very small. If 

 the genes are far enough apart, the recombination classes 

 may be nearly as large as the parental combination classes. 

 This phenomenon of linkage and crossing over is of extreme 

 importance in plant and animal breeding. 



In connection with the problem of linkage it would be 

 well to mention the backcross. The backcross is a very 

 powerful tool for the plant or animal breeder. To illustrate 

 a case, let us suppose that a breeder finds a variety of sweet 

 pea which is fairly resistant to stem streak, a virus disease 

 of sweet peas. He wishes to introduce this valuable charac- 

 teristic into a particular variety of Late Spencer sweet peas 

 which has particularly nice color and flower shape. The pro- 

 cedure would be to cross the two varieties of sweet pea. The 

 Fi secured is then backcrossed to the Late Spencer. The 

 progeny of this backcross are examined and those plants 

 which resemble the Late Spencer most closely and also have 

 the character for disease resistance are again backcrossed to 

 the Late Spencer parent. Again the progeny are selected 

 and again the backcross is made. If the process is repeated 

 long enough, it is theoretically possible to produce a new 

 variety with all the flower characteristics of the desired Late 

 Spencer plus the disease resistance. This is possible even if 

 the disease resistance is closely linked with some of the genes 

 for undesirable flower characteristics. If linkage is close, 

 however, the process of selection and backcrossing must be 

 carried on longer and with a larger number of selections for 

 each generation. This is possible because crossing over 

 taking place at random will eventually free the character for 

 disease resistance from the other genes. Selection must be 

 carried out carefully, however, and artificial inoculation of 

 the virus disease is desirable in order to be sure that the selec- 



