118 GENETICS AND EUGENICS 



same reason, lack of the color factor. Together the albinos 

 number four. 



A different modification of the typical dihybrid ratio is 

 illustrated by the following case in which two varieties were 

 crossed which possessed complementary factors neither of 

 which is able to produce a visible effect apart from the other. 

 When certain white-flowered varieties of sweet peas are 

 crossed with each other they produce Fi plants which bear 

 red-colored flowers (Bateson and Punnett). F2 consists of two 

 apparent varieties only, viz., reds and whites in the ratio, 

 nine red to seven white. This is explained as a modified di- 

 hybrid ratio (9:3:3:1) in which the last three terms are in- 

 distinguishable (all being white) . The two factors involved 

 in this case are assumed to be a color factor found in one white 

 parent and a red factor found in the other, both together (in 

 Fi) producing a red color, but either by itself producing no 

 color whatever. One parent accordingly produces gametes 

 all Cr, the other produces gametes all cR. Fi is CcRr, a 

 double heterozygote ; its gametes, CR + Cr + cR + cr; 

 and the F2 zygotes containing the same assortments of fac- 

 tors are 9CR:3Cr:3cR :lcr. But if C and R, neither of 

 them, produce color apart from each other, then only the 

 9 CR zygotes are colored, all the others, seven in sixteen, 

 being white, and the observed F2 ratio (9 : 7) is thus accounted 

 for as the result of a dihybrid cross at the same time that the 

 Fi result is explained. 



When some other white-flowered varieties of sweet peas 

 are crossed with each other, there are produced, not red- 

 flowered Fi plants as in the foregoing case, but those which 

 are purple bi-color, like the wild sweet pea, a case of rever- 

 sion or atavism, like those known for pigeons, rabbits and 

 guinea-pigs. This reversion involves a third independent 

 factor (a factor for blue, B) which is ineffective except in 

 the presence of both the color factor (C) and the red factor 

 (R). When in such reversionary crosses a colored Fi is pro- 

 duced which is heterozygous for all three factors, F2 mani- 

 fests a peculiar modified trihybrid ratio, less common than 



