284 



THE SECOND-CHROMOSOME GROUP 



Nearly all of our other dominant autosomal mutants are lethal when 

 homozygous, and therefore give 2 : 1 ratios when inbred, as in the type 

 case of the yellow mouse. 



DESCRIPTION OF SQUAT. 



The squat flies seen in culture 2636, and in subsequent cultures, may 

 be more exactly described by aid of the drawing of the somewhat 

 atypical specimen of figure 85. The most striking change, and the one 

 most dependable in classification, is that of the wing, which is about 

 80 per cent the normal length, is slightly broader than normal, and has 

 a blunt end. The whole wing is of a somewhat weaker texture, with a 

 tendency to droop like "arc." The color of the wing is somewhat 

 cloudy and brownish instead of the clear gray of the normal. The 

 wings are also sometimes slightly divergent. The thorax is short, 

 broad, and rather 

 flattened on top. 

 The head likewise 

 is broad from side 

 to side, and quite 

 often the eye has 

 a protruding lump 

 which is caused by 

 an extra antenna 

 pushing partly or 

 entirely through. 

 The legs are weak 

 and shortened, 

 especially in the 

 basal joints. None 

 of the above 

 changes are very 

 marked or of uni- 

 form occurrence. 

 One learns to recog- 

 nize the type much as in the case of certain wild species, or Oenothera 

 mutants, by the ensemble of slight differences. 



CHROMOSOME OF SQUAT. 



To test whether squat is third-chromosome or not, squat males 

 were out-crossed to the third-chromosome dominant dichsete. Both 

 Fi pairs (2730, 2829) gave very few squats, and these difficult of class- 

 ification. It seemed, and this has proven to be the case, that the squat 

 character behaves as does truncate and several other of our mutations. 

 Presumably, like truncate, it owes most of this evanescence to its 

 extreme sensitiveness to modification, both in intensification and sup- 

 pression, brought about by the action of different combinations of 



Text-figure 85. — Squat. 



