MODIFIEt) F2 RATIOS 119 



the modified dihybrid ratios just discussed. If, for example, 

 one white parent contributes the color factor while the other 

 parent contributes the red and the blue factors, then we may 

 represent the parental gametes as Crb and cRB respectively. 

 Fi will then be a triple heterozygote, CcRrBb, which from 

 the combined action of the three dominant characters will be 

 a purple bi-color. Its gametes will then be of eight sorts and 

 the zygotes in which corresponding groupings of the domi- 

 nant factors occur will be as follows: ^ 



27 CRB, purple 3 Crb, white 



' 9 CRb, red 3 cRb, white 



9 CrB, white 3 crB, white 



9 cRB, white 1 crb, white 



But only the first two of these eight groupings contain com- 

 binations of factors capable of producing colored flowers, viz., 

 CRB, which will produce purples, and CRb, which will pro- 

 duce reds. All the other six combinations lack one or both 

 of the two factors (C and R) which must be present together 

 in order to produce colored flowers. Consequently all will 

 produce uncolored (white) flowers, and the expected classes 

 of phenotypes will be as follows: twenty-seven purple, nine 

 red, twenty-eight white, a modified trihybrid ratio. 



Summary on Modified Ratios 



1. When a cross involves two factors, one of which pro- 

 duces no visible effect except in the presence of the other, 

 the dihybrid F2 ratio, 9:3:3:1, is modified to 9:3:4, because 

 the last two classes of the typical ratio are indistinguishable. 



2. WJien a cross involves two factors, rieither of which pro- 

 duces a visible result in the absence of the other, the dihybrid 

 ratio becomes 9 : 7, because the last three classes of the 

 typical ratio are indistinguishable; if in addition a third 

 factor is involved which prockices no visible effect except in 

 the presence of both the others, a modified trihybrid ratio 

 is obtained, viz., 27:9:28. 



^ It is suggested that the reader make out the trihybrid checkerboard calcula- 

 tion for this cross and color the squares with crayon in accordance with the assump- 

 tion made. 



