75 



Velvet chaff being dominant over smooth chaff, the first generation 

 hybrids will be velvet-chaffed, but in the second and succeeding generations 

 both velvet and smooth-chaffed forms will appear. The latter being recessive 

 will breed true; the former, or a certain proportion thereof, will continue 

 to segregate into velvet and smooth-chaffed forms. It is therefore a difficult 

 task, requiring several years of careful " rogueing " to get rid of all velvet- 

 chaffed forms. 



The persistent reappearance of white-chaffed forms in apparently pure 

 field cultures of brown-chaffed sorts, bearded forms in fields producing bald 

 sorts and white-kernelled forms in red or black sorts also seems capable of 

 explanation, in part at least, as due to the biological difficulty of permanently 

 removing all dominant heterozygotes. Thus, when a white and a black oat 

 sort become crossed, the first generation hybrids are black as black is dominant 

 over white. In the second and succeeding generations both Black and White 

 grained forms arise. White being recessive the forms possessing this color 

 will breed true; black being dominant, heterozygous forms are concealed 

 which in following generations, divide up into whites and blacks as well as 

 into other forms presenting intermediate shades or gradations of color. 

 Thus will be seen the impossibility of entirely removing white-kernelled 

 forms from an inconstant black-kernelled sort by a single picking out of all 

 white kernels. This is illustrated more clearly in the following diagram 

 which shows what may be expected in each of seven generations from one 

 kilogram (about 30,000 kernels) of .a black oat sort of which 128 kernels are 

 heterozygous (41, p. 235). 



Black Heterozygotes White 



29872 128 



29,904=29,872 + 32 64 32 = 32 



29,920 = 29,872 + 32 + 16 32 16 + 32 = 48 



29,928 = 29,872 + 32+16 + 8 16 8 + 16 + 32 = 56 



29,932 = 29,872 + 32 + 16 + 8 + 4 8 4 + 8 + 16 +32 = 60 



29.934 = 29,872 + 32 + 16 + 8 + 4 + 2 4 2 + 4 + 8 + 16 +32 = 62 



29.935 = 29,872 + 32 + 16 + 8 + 4 + 2 + 1, 2 1 +2 + 4 + 8 + 16 +32 = 63 



By the regular Mendelian segregation of the inconstant forms (Hetero- 

 zygotes) the number of constant white-kernelled forms has increased from 

 32 to 63 in seven generations. From this time onward the production of such 

 forms will be negligible in so far as the crossing in question is concerned. By 

 continually picking out white-kernelled individuals as they appear, the 

 number is ultimately reduced until practically none remain. A couple of 

 instances may here be given: In the Black sort, Great Mogul, there were 



