REVERSION 



6i 



it lay in following the results flowing- from crosses 

 between blacks and whites. Experiments recently 

 made by Staples-Browne have shown that this case 

 of reversion also can be readily interpreted in 

 Mendelian terms. In these experiments the cross 

 was made between black barbs and white fantails. 

 The Fj birds were all black with some white 

 splashes, evidently due to a separate factor intro- 

 duced by the fan- 

 tail. On breeding 

 these blacks to- 

 gether they gave 

 an Fg generation, 

 consisting of blacks 

 (with or without 

 white splashes), 

 blues (with or 

 without white 

 splashes), and 

 whites in the ratio 

 9:3:4. The factors 

 concerned are 

 colour (Q, in the 

 absence of which 

 a bird is white, and 

 a black modifier {B),in the absence of which a coloured 

 bird is blue. The original black barb contained both 

 of these factors, being in constitution CCBB. The 

 fantail, however, contained neither, and was con- 

 stitutionally cchb. The F^ birds produced by 

 crossing were in constitution CcBb, and being 

 heterozygous for two factors produced in equal 

 numbers the four sorts of gametes CB, Cb, cB, cb. 



Fig, II. 



Diagram to illustrate the appearance of the rever- 

 sionary blue pigeon in Fg from the cross of black 

 with white. 



