278 HEREDITY AND ENVIRONMENT 



primary and secondary sexual characters, but 

 they may appear in either sex though they are 

 usually transmitted from fathers to daughters, 

 or from mothers to sons ("criss-cross" inheri- 

 tance) in exactly the way in which the sex 

 chromosomes (X) are transmitted. Morgan 

 has therefore concluded that the factors for 

 these characters are carried by the sex chromo- 

 somes and has named them sex-linked charac- 

 ters. In the fruit fly, Drosophila, he has 

 discovered more than twenty-five such charac- 

 ters, applying to the color of the eyes and of 

 the body, to the length of the wings, etc. A 

 typical case is shown in Figs. 59 and 60. The 

 eye color of this fly is normally red, but muta- 

 tions have arisen in which the eye is white. 

 Such a mutation always appears in males, 

 though it may later be transferred to females, 

 as we shall see. If now a white-eyed male 

 and a red-e} r ed female are crossed all the F x s 

 are red-eyed, but if these F x s are interbred all 

 the females of F 2 have red eyes while half of 

 the males have red eyes and the other half 

 have white eyes (Fig. 59) . On the other hand 

 if one of the F x females of this cross is bred 



