HYBRIDISATION AND WHEAT HYBRIDS 369 



COLOUR OF THE GLUMES. The glumes of wheat are generally described 

 as some shade of black, red, or white, the red embracing various shades of 

 brown or red, and the white various shades of yellowish- white. Black 

 and red are more or less dominant over white, and black also dominates 

 the red tints. 



i. Black* Red. According to Tschermak, white-chaffed individuals 

 sometimes appear among the descendants of this cross, and Rimpau 

 obtained white-glumed forms in the F 2 generation of a cross between a 

 bluish-black chaffed T. turgidum and a red-chaffed T. vulgare (p. 400). 



ii. Black x White. Tschermak states that the F 2 of this cross is of 

 complicated character, but the ratio of the pigmented to the white forms 

 is 3 : i. 



From a cross of Black Emmer (T. dicoccum. var. atratum) xa white- 

 chaffed T. sphaerococcum I obtained in F 2 forms with the following chaff 

 colours : uniform black, red, and white, and chaff of these tints with black 

 outer margins. A similar series was obtained by Kezer and Boyack in 

 the F 2 of the hybrid Black Emmer xa white-chaffed T. vulgare (p. 393). 



iii. Red x Red. The descendants of this cross are generally some shade 

 of red, though, as indicated later, white individuals may arise in F 2 from 

 the hybridisation of two red forms. 



iv. Red x White. Very numerous hybrids have been raised between 

 red-chaffed wheats and white-chaffed wheats : in the majority of these 

 cases the F x is intermediate in colour, and the segregation in the F 2 genera- 

 tion of the ordinary monohybrid type 3 red : i white, or i red : 2 pale 

 red : i white, the colour being dependent upon a single pair of Mendelian 

 factors. In such wheats each of the many shades of red appears to have 

 its own special factor. 



In crosses made between white wheats and some red-chaffed Swedish 

 " Landwheats " Nilsson-Ehle found that the segregation in F 2 was that 

 of a dihybrid cross, viz. 15 red : i white. 



He showed that the red colour in these cases is not due to a simple 

 factor but to two different and separately inherited red factors ; each of 

 these by itself produces a red tint, not necessarily of the same intensity, 

 and when acting together they have an additive or cumulative effect, the 

 colour increasing with the number of doses of the colour factors present 

 in the zygote. 



If the two red factors are represented by R x and R 2 and their absences 

 by r, and r 2 respectively, a red-chaffed wheat (RiR 2 ) when crossed with a 

 white-chaffed variety (r 1 r 2 ) results in a heterozygote Fj of the constitution 



R 1 R 2 r i r 2- 



The sixteen possible zygotes of the F 2 generation with their respective 

 doses of red factors are shown in the following diagram : 



2E 



