176 



GENETICS IN RELATION TO AGRICULTURE 



in some locus concerned in the development of height, such that a low 

 prostrate form was produced rather than the normal tall form. We may 

 call this locus D and its mutated condition in Cupid sweet peas d (dwarf). 

 In the bush sweet pea a change must have occurred in some other locus 

 concerned in the development of height such that the bush type was 

 produced rather than the normal tall sparsely branched type. This 

 locus in the germinal material may be called B, and its mutated con- 

 dition responsible for the bush type of growth, b (bush). Considering 



Fio. 83. Checkerboard analysis of the Ft population resulting from a cross between bush 



and Cupid sweet peas. 



both loci, the tall form must have the genetic constitution DDBB, since 

 a change in either one of these factors results in some other type than 

 the normal tall. The bush will then obviously be DDbb, since it shows 

 only a single factor difference from tall, and likewise the Cupid sweet 

 pea will be ddBB. When, therefore, bush is crossed with Cupid, the 

 genetic formula for the FI is DdBb. Since the factor combination nec- 

 essary for the production of tall plants is reconstituted in this cross, the 

 F\ plants are all tall. In F z segregation takes place in accordance with 

 expectations based on independent distribution of the two pairs of 

 allelomorphs as shown in the accompanying checkerboard in Fig. 83. 



The F 2 segregation here displayed is in agreement with the usual 

 dihybrid ratio when dominance is complete. There are 9 tall: 3 bush: 3 

 Cupid :1 bush-Cupid. In the bush-Cupid a new combination, of factors 



