148 



GENETICS IN RELATION TO AGRICULTURE 



The various homozygous combinations of these two factors give four 

 primary races which breed true as follows: 



YYRR = crimson. 



YYrr = yellow. 



yyRR = white. 



yyrr =. white. 



By hybridizing these races four heterozygous forms may be produced 

 which are of the colors given below: 



YYRR (crimson) X YYrr (yellow) gives YYRr = orange red. 

 YYRR (crimson) X yyRR (white) gives YyRR = magenta. 

 YYRR (crimson) X yyrr (white) gives YyRr = magenta-rose. 

 YYrr (yellow) X yyrr (white) gives Yyrr = pale yellow. 



YR 



Yr 



yR 



yr 



Fio. 68. Checkerboard analysis of the progeny of a magenta-rose Mirabilis of the genetic 



constitution YyRr. 



We thus have seven distinct color classes as a result of various com- 

 binations of two pairs of color factors. 



Moreover, this species gives a very good example of the diversity 

 which may be obtained in an F z population. Thus Miss Marryat 

 has shown that when magenta-rose, YyRr, is selfed, the progeny fulfil 

 the conditions indicated by the accompanying checkerboard analysis 

 in Fig. 68. 



TABLE XXIX. F t PHENOTYPES AND F 3 PHENOTYPIC RATIOS DERIVED FROM 

 THE ORIGINAL CROSS, CRIMSON, YYRR X WHITE, yyrr 



