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 
DDBB DDBb DdBB DdBb 
tall tall tall tall 
all tall 3 tall : 1 bush 3 tall : 1 Cupid 9 tall : 3 bush: 
3 Cupid: 
1 bush-Cupid 
DDBb DDbb DdBb Ddbb 
3 tall : 1 bush all bush 9 tall : 3 bush: 3 bush: 
3 Cupid: 1 bush-Cupid 
1 bush-Cupid 
DdBB DdBb ddBB ddBb 
tall tall Cupid Cupid 
3 tall: 1 Cupid 9 tall : 3 bush: all Cupid 3 Cupid: 
3 Cupid: 1 bush-Cupid 
1 bush-Cupid 
DdBb Ddbb ddBb ddbb 
tall bush Cupid bush-Cupid 
3 Cupid: 3 bush: 3 Cupid: all bush-Cupid 
1 bush-Cupid 1 bush-Cupid 1 bush-Cupid 
Fic. 83.—Checkerboard analysis of the F2 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 F, 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 F2 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, 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 
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