90 GENETICS IN RELATION TO AGRICULTURE 
The males, however, produce the following series of sperms: 
V(WX) v(WX) VY. vY. 
Mating F flies of this cross, therefore, results in the Ff; population shown 
in the checkerboard in Fig. 44. When these are collected into like pheno- 
types the ratio obtained is 9 long red:3 long white:3 vestigial red: 
1 vestigial white, but this agreement with the typical dihybrid ratio is only 
apparent. When the females alone are considered the ratio is 6 long red: 
2 vestigial red, and the males are in the ratio 3 long red:3 long white: 
1 vestigial red:1 vestigial white. The disturbance in the ratio is 
occasioned by the unique behavior of the white-eye character which 
behaves exactly as it did in the simple case which was analyzed previously. 
The reciprocal ratio, therefore, is additional evidence as to the adequacy 
of the chromosome theory. 
The Trihybrid.—The same line of reasoning of course applies to 
cases in which three pairs of factors are involved. Such for example is a 
case which Baur has described in the common garden snapdragon, 
Antirrhinum majus. In this particular case the factors involved have 
the following relations. 
Z—a factor which conditions the development of the zygomorphic 
type of blossom which is characteristic of the species. The factor z, its 
allelomorph, conditions the production of peloric blossoms, 7.e., blossoms 
which display radial symmetry. The normal form is nearly completely 
dominant. 
R—a factor for red color of the blossoms. The allelomorph r gives 
flowers which are flesh colored. 
I—a factor for ivory coloration of the blossoms. The allelomorph 
7 in this case conditions the production of yellow flowers. R with I gives 
flowers red on an ivory background, a magenta type of coloration, 
whereas R with 7 gives flowers which are red on a yellowish background. 
It is possible to distinguish these two classes in a mixed population. 
Tasie XIII.—Distrmvution or Ciasses AMONG THE PROGENY oF an F, Hysrip 
SNAPDRAGON OF THE Composition ZzRrli 
Ratio Phenotypes Observed Expected 
Qi, Zygomorphie, red on ivory.................. 64 | 52 
9 Zygomorphic, red on yellow................ 14 17 
9 Zygomorphic, flesh-colored, on ivory......... 10 17 
3 Zygomorphic, flesh-colored, on yellow........ 23 17 
9 Pelorie; red: ODAVOLyace acs acces skeaes eee 6 6 
3 Peloric, red, on yellow..................45. 1 6 
3 Peloric, flesh-colored, on ivory.............. 4 6 
1 Peloric, flesh-colored, on yellow............. 2 2 
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