UNIT-CHARACTERS OF INSECTS 157 



ence for male individuals, which has led to its being called a 

 sex-linked character. White-eye has proved to be only the 

 first of a long series of unit-character variations, which have 

 appeared in Professor Morgan's cultures of Drosophila, 

 which have this same curious sex-linked character. Among 

 these may be mentioned a variation in which the entire body 

 is yellow, another in which the eye-color instead of being an 

 ordinary red, is a brilliant vermilion, and several variations in 

 the form of the wing known as rudimentary, miniature, 

 forked, etc. It is found that when a race possessing two of 

 these recessive sex-linked characters (as white eye and yellow 

 body) is crossed with another race which lacks them, there is 

 a tendency for the two sex-linked characters to go together 

 in heredity, so that whatever F2 individuals possess one of 

 them possess also the other. This suggests that the material 

 basis or "gene" of each lies in the germ-cell near that of the 

 other, that their genes are either connected directly with each 

 other or with a common third structure. Since there are 

 several of these variations which show "linkage" with each 

 other and a peculiar relationship to sex, the pertinent sug- 

 gestion was made by Morgan that they had as a common 

 connecting element a structure concerned in the determina- 

 tion of sex, commonly known as the sex-chromose or X-chro- 

 mosome. The "genes" of sex-linked characters, according 

 to Morgan, lie in the X-chromosome and the peculiar features 

 of the inheritance are due to the fact that the X-chromosome 

 is paired in females but unpaired in males. Strong support 

 is given to this idea by the result of crosses in which each 

 parent introduces a different sex-linked character, as in the 

 cross between a white-eyed race and a yellow-bodied race, 

 each being otherwise normal. The two characters in this 

 case keep apart as strongly as they keep together when in- 

 troduced into a cross by the same parent. This is exactly 

 what we should expect if, as Morgan supposes, sex-linked 

 characters have their genes in a common cell structure (for 

 example an X-chromosome). For when two genes lie in the 

 same X-chromosome, they will go together (show linkage). 



