MENDELIAN SEGREGATION 13 



istic Mendelian ratio when one pair of characters is 

 involved. -- — 



In a third-chromosome stock, ebony, the body and 

 wings are very dark in contrast to the wild fly whose 

 color is "gray." Gray is used to designate the color 

 of the wild fly, whose wings are gray, but whose 

 body is yellowish with black bands on the abdomen. 

 If ebony is crossed to gray the offspring (Fi) are gray 

 but are somewhat darker than the ordinary wild flies. 

 When these hybrids are inbred they give (F2) 1 gray, 

 to 2 intermediates, to 1 ebony. The group of inter- 

 mediates in the second generation (F2) can not be 

 separated accurately from the pure gray type. If they 

 are counted as gray, the result is three grays to one 

 ebony. 



Since ebony and gray assort independently of long 

 and vestigial, as will be shown later, the factor for 

 ebony must be supposed to be carried by a chromo- 

 some of a different pair from the one that carries 

 vestigial. Since this chromosome behaves in the 

 same way as does the one that bears the vestigial 

 factor, the scheme used for vestigial will apply here 

 also. 



Another mutant stock is characterized by small 

 eyes, and since in the extreme form it may lack one 

 or both eyes entirely (Fig. 7), the name "eyeless" 

 has been given to this mutant. When this stock is 

 bred to wild flies the offspring have normal eyes. 

 These inbred give three normal to one eyeless fly. 

 As shown in the table on page 6, this character 

 belongs in still another, the fourth, group, and its 



