36 Colour and other Characters in the Potato 



The results^ are sufficiently close to give one some confidence that the 

 phenomena are correctly represented by the assumption of the factors 

 PR and D that have been supposed to be at work. 



The sister family K^ adds additional evidence of a strong nature. 

 Several lots of seed of K^ plants were sown and in all some 300 seedlings 

 raised. The majority were however planted in selections and therefore 

 are of no use for quantitative purposes. All the groups, however, 

 coincided in one feature — none produced a single red tuber; and the 

 evidence from the selected groups strongly favour the view that purples 

 to whites were as 9:7, whilst the groups that were planted in full 

 give 26 : 14. The parent plant of such a family must be homozygous 

 in the purpling factor and heterozygous in its two other colour factors. 

 To K^, therefore, should be given the zygotic formula PP, Mr, Dd. 



Having considered K^ aod K^, we can now turn back to the original 

 cross and the F^ family. The F^ family consisted of 13 purple, 12 red, 

 4 white. It is obvious that as regards P, " Congo " must be heter- 

 ozygous, further we knew the " Flourball " seedling was red and 

 therefore contained RD. If we represent the cross 



" Congo " PpRrDD x " Flourball " seedling RrDD 



we get 12 purple, 12 red, 8 white. 



The result of these experiments on colour inheritance would seem to be 

 (1) that whilst colour may be present in the stem to any degree, a 

 special developer D is necessary to bring it out in the tuber, (2) that 

 redness is dependent on a separate factor R, (3) that purple is 

 dependent on a further one P, and (4) that the purple colour cannot 

 be developed except in the presence of all three factors PRD. 



In all the experiments there has been much to suggest that the 

 degree of the " redness " is due to the homozygous condition or other- 

 wise of the plant as regards both R and D, but the evidence has not 

 been given in full because the classification into shades of " redness " 

 would be too empirical and dependent on personal judgment. In one 

 group the distinction was clearly made out, viz. in the family A where 

 the formula was shown to be RrDD (or RRDd) the deep reds were to 

 the remaining reds as 24 to 48, whilst in the K^ group the reds were 

 23 deep red to 6 pale red. Amongst the blacks (purple) no distinction 

 could be made. 



1 If the disproportionate mortality of the whites be remembered, the actual numbers 

 will be seen to be not so far removed from the calculated ones. Thus the number of 

 whites, had the mortality in all classes been equal, would be 66 instead of 54. 



