218 Blakeslee. 



Black Yellows with Red Yellows. The results are shown in Table 6 a. 

 All the Fl plants have purple cones. 



The g-enes for yellow cones are obviously in different chromosomes 

 in the two groups. The two kinds of yellows shoukl therefore segregate 

 independently in the F2 generation. This they do as is shown by the 

 records in Table 6b. The 9:3:3:1 dihybrid ratio becomes a 9:7 

 ratio if merely the natural characters, purple and yellow, are considered. 

 As in the monohybrid ratios the yellows are somewhat below expectation, 

 although the deviation is not quite three times the probable error. 



When the yellow cones are treated with KOH, the two types, 

 Black Yellows and Red Yellows are distinguishable. It has not been 

 possible to identify the double recessive, homozygous for both types of 

 yellow by the chemical test applied. It can be assumed to be in- 

 distinguishable from the Red Yellows, since this class predominates in 

 numbers over the Black Yellows. This being the case, the dihybrid 

 ratio becomes resolved into a 9:4:3 ratio. Of the tested yellows in 

 the F2 generation there are 52 RY" to 43 BY^ when the expectation in 

 a 4 : 3 ratio would be 54-3 : 40"7 + 3'25. The agreement is close since 

 the deviation is less than the probable error. 



Plate 9 is a diagram illustrating the dihybrid ratio obtained when 

 the two yellow cone colors are crossed and bred to the F^ generation. 

 The cones in parentheses show the colors which the adjacent yellow 

 cones assume when treated with KOH. 



The independence of the genes for Black Yellow and for Red 

 Yellow, shown by the Fi and F2 generations, illustrated in the colored 

 diagram, is confirmed by back crosses of the Fi with the two recessives. 

 The results, tabulated in Table 6c agree closely with expectation. The 

 double recessive, homozygous for both BY and RY" has not been identified. 

 It would theoretically be an easy task, by back-crossing the yellow 

 coned individuals of the F-2 generation with BYs and RY^s, to find 

 plants which would give yellow coned offspring with both types. 



In the foregoing paragraphs it has been assumed that the double 

 recessive is indistinguishable from the Red Yellows since the latter class 

 predominates in the F,. generation. In as much as the Black Yellows 

 show a somewhat greater deficiency in F« and back crosses than the 

 Red Yellows, it is possible that the condition is reversed and the Black 

 Yellows include the double recessive in the Fo pedigrees. The deviation 

 from the expectation that the Black Yellows and the Red Yellows form 

 a 4 : 3 ratio is only slightly over three times the probable error. 



