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 was 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 always 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 
3 
