232 



THE SECOND-CHROMOSOME GROUP 



"spineless" was not also "morula," the conclusion that they are 

 effects of the same gene is justified. The name "morula" was then 

 retained, and the name "spineless" was given to the third-chromosome 

 recessive which now bears it. 



The "spineless" flies must have had the "morula" character at the 

 time of their discovery, and this character must have passed unno- 

 ticed for several generations, during which several hundred such flies 

 were examined. That such a conspicuous character should be present 

 and be overlooked would be almost unbelievable were it not that many 

 other similar failures have been made, notably in the cases of dachs 

 (see p. 216) and club. In the case of club, Morgan found and worked 

 with the wing-character for many generations without discovering the 

 absence of the pleural bristles which is the constant index of the mutant, 

 Bridges surpassed this by finding in another stock this same mutant, 

 which he worked with as a bristle character, and entirely failed to 

 recognize the far more conspicuous wing modification which was present. 



CHROMOSOME CARRYING MORULA. 



To determine whether the gene for "morula" was in the second 

 chromosome, a morula male was out-crossed to a curved female and 

 three F2 pair cultures raised (table 78) . 



Table 78. — Pi, morula cf X curved 9 ; Fi wild-type 9 -\- Fx wild-type cf . 



The F2 ratio was plainly a 2 : 1 : 1 : ratio, with no double recessives, 

 which proved that morula was in the second chromosome. 



An attempt to obtain the curved morula double recessive from F3 

 or F4 matings failed, but in F4 from a similar cross of morula to black 

 the double recessive was readily obtained. 



LOCUS OF MORULA. 



From this difference in the ease of obtaining the double recessive it 

 was suspected that morula was nearer to curved than to black in the 

 second chromosome. 



It became apparent in F3 that the double recessive black morula 

 would be obtained in F4, since some of the F2 black females and morula 

 males mated together had produced in F3 a few black flies which were 

 then known to be heterozygous for morula and which must produce 

 black morula offspring among their progeny when inbred. Accord- 

 ingly, by making an Fi mating of morula male by black female at the 



