HYBRIDISATION AND WHEAT HYBRIDS 373 



Red dominates white, the F x generation being red-grained, but the 

 grains are a paler tint than those of the red parent. 



The segregation in F 2 depends on the character of the red parent. 

 In a large number of instances the F 2 generation consists of red- and 

 white-grained descendants in the proportion of 3 red : i white, the red 

 tint being due to a single Mendelian factor. 



Nilsson-Ehle discovered that the trihybrid ratio of 63 red : i white 

 occurred in the F 2 generation when certain red wheats were crossed with 

 white. 



From the hybrid Svalof " Pudel " 9 (a white velvet-chaffed, white- 

 grained Squarehead) x Swedish " Sammet " <$ (a velvet-chaffed form 

 with dark red grain) he obtained in F x plants with pale red grains. All 

 the grains of the F 2 generation were also red, but of several shades. On 

 raising the F 3 generation from one of these hybrids, he found that some of 

 the plants segregated approximately in the ratio 3 red : i white ; others 

 gave a progeny of about 15 red : i white, while a smaller number exhibited 

 the trihybrid segregation of 63 red : i white. 



An analysis of the results led him to the conclusion that the red grain 

 colour of the particular parent employed was dependent upon the presence 

 of three dominant and independent factors R x , R 2 , and R 3 , each of which 

 by itself is capable of producing redness, the varying shades of grain tint 

 among the progeny of the cross with a white-grained parent (rjr 2 r 3 ) in 

 which these factors are absent, being brought about by the additive or 

 cumulative action of the several colour factors. For example, where two 

 or three factors are present in the zygote the red tint of the grain is twice 

 or thrice the depth of colour of those of a plant into which only one has 

 entered. Only when all the factors are present is the original dark red- 

 tinted grain of the parent reproduced. 



On this hypothesis the gametic constitution of the red parent is 

 R X R X R 2 R 2 R 3 R 3 , that of the contrasted white parent r^ r 2 r 2 r 3 r 3 . The 

 sixty-four possible individuals and twenty-seven genotypes of the F 2 

 generation obtained from the cross RjR-jRg x r 1 r 2 r 3 may be arranged as 

 shown in table on top of next page. 



The number of red-producing factors entering into the various geno- 

 types of the F 2 generation is indicated at the base of each column of the 

 diagram ; the frequency distribution of the numbers of individuals of 

 the several tints of the population is that of a symmetrical polygon of 

 fluctuating variation. 



