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). 



