Cases of uiimic mutations and secondary mutations in the X-chroniosome v*ic, 



15 



character deseribecl, it has beeu showD that they are iieveitheless 

 geuticaUy uurelated. Not ouly tliey have different loci in tiie sex 

 chromosome, but one of them causes a complete sex -limited sterility, 

 while the other does not affect the fertility of the fly. 



In order to study the effect of singed and forked^ when com- 

 bined in double recessive singed forked* flies, females which carried 

 eosin singed in one X and vermilion 



vy6 gn 



forked* in the other ( .. 



V f*) 



were crossed singly to eosin ver- 

 milion forked males (2599, 2600: 

 April 12, 1921). Forked males 

 were used for the purpose of 



f 

 obtaining in the same experiment .^ 



females so that this compound 

 could be compared with the singed, 

 the forked* and the double re- 

 cessive singed forked* character 

 in flies which were derived from 

 the same parents. All the latter 

 classes occur among the males. 

 Half of the original combination 

 males will be eosin singed and the 

 other half vermilion forked*. Of 

 the eosin males a considerable per- 

 centage will be eosin singed forked* 

 as a result of crossing -over bet- 

 ween vermilion and forked*, and 



the eosin vermilion males will be of the constitution eosin vermilion 

 forked * resulting from crossing-over between eosin and singed or else eosin 

 singed vermilion forked* from crossing-over between singed and Vermilion. 

 In spite of careful examination it was found to be impossible to 

 separate the singed forked* flies from pure singed or pure forked* 

 respectively, the character being not exaggerated in double recessive 

 singed forked* flies. With regard to the female f — f* compound this 

 was more like forked* than like ordinary forked, the hair and bristle 

 alteration of the compound being not quite as marked as that of homo- 

 zygous forked* individuals. 



Fig. .3. Forked* male. 



