TYPES OF MENDELIAN HEREDITY 



33 



flies even in a pure stock of club show this character, 

 it was difficult to study the inheritance of the 

 supposed factor that sometimes inhibits the unfolding 

 of the wing pads. Nevertheless, it w^as possible 

 even with this handicap to show that the character 

 depended on a sex linked recessive factor. Later 



Fig. 17. — Club wing (to left). The absence of the spines on the side 

 of the thorax in "club" is shown in c, and the normal condition is shown 

 in b. 



the discovery was made that a particular pair of 

 spines ahvays present on the side of the thorax of 

 the wild flies, is absent from the club flies, irrespective 

 of whether the wings do or do not unfold (Fig. 17, c). 

 This constant feature of the mutant made its study 

 quite simple. Another pair of spines, those upon the 



