182 Heredity and Environment 



studied by Morgan were in characters which are usually asso- 

 ciated with one or the other sex, but which may have nothing to 

 do with reproduction and may affect any part of the body. Such 

 characters are not necessarily limited to one sex or the other, as 

 are many 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" in- 

 heritance) 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 chromosomes and has 

 named them sex-linked characters. In the fruit fly, Drosophila, 

 he has discovered more than twenty-five such characters which 

 are linked with sex, such as the color of the eyes and of the body, 

 the length of the wings, etc. A typical case is shown in Figs. 61 

 and 62. The eye color of this fly is normally red, but mutations 

 have arisen in which the eye is white. Such a mutation always ap- 

 pears in males, though it may later be transferred to females, as 

 we shall see. If now a white-eyed male and a red-eyed female are 

 crossed all the /Vs are red-eyed, but if these F^s are interbred all 

 the females of F z have red eyes while half of the males have red 

 eyes and the other half have white eyes (Fig. 61). On the other 

 hand if one of the F t females of this cross is bred with a white- 

 eyed male half of the females of F 2 are red-eyed and half are 

 white-eyed, and half of the males are red-eyed and half are white- 

 eyed. 



If now one of these white-eyed females is bred with a red-eyed 

 male all the females of the F i generation are red-eyed and all the 

 males white-eyed ("criss-cross" inheritance) and if these are 

 interbred there are produced in the F 2 generation equal numbers 

 of red-eyed and white-eyed males and females (Fig. 62). 



The distribution of the maternal and paternal sex chromo 

 somes* exactly parallels this distribution of this sex-linked char- 



* Morgan has found that there are two different sex chromosomes (XY) 

 in the male Drosophila, instead of one (XO) as Stevens supposed; the Y is 

 found only in males. 



