344 CALVIN B. BRIDGES 



That the action of cream II is specific only to eosiii was sug- 

 gested by crosses of cream with vermihon (x chromosome) and 

 with pink (third chromosome). A careful examination of the Fo 

 flies from these crosses showed no dilution of ether vermilion or 

 pink by the creaiji, that is, the double recessive vermilion cream 

 and pink cream (not-eosin) are indistinguishable from vermilion 

 and pink, respectively. 



The proper method of study for cases of multiple factors or of 

 modifiers is by means of linkage experiments, whereby all guess- 

 work as to the number and effect of modifiers can be eliminated. 

 In Drosophila such a study is rendered particularly easy by the 

 small number of chromosomes and by the fact that in the male 

 there is no crossing over of any of the chromosomes. In conse- 

 quence, if two recessive genes which belong to the same chromo- 

 some, for example, black and vestigial of the second chromosome, 

 enter the cross from opposite parents ('repulsion') the Fo never 

 shows flies which have both these mutants at the same time. The 

 double recessive class is entirely unrepresented, and the 2:1:1:0 

 ratio of 'absolute repulsion' results. This ratio holds whatever 

 maj' be the amount of crossing over in the female, for the lack of 

 double recessive sperm prevents the double recessive eggs from 

 revealing themselves. This ratio is in marked contrast to the 

 9:3:3:1 ratio which obtains when the two genes belong to 

 different chromosomes, for example, curved of the second chromo- 

 some and ebonj' of the third chromosome. 



The light color cream was known to be eosin plus a recessive 

 modifier which belonged to an autosome linkage group. To find 

 whether this group was that of the second chromosome, a cream 

 male (from pure stock) was outcrossed to a curved female, curved 

 being a recessive wing character whose gene is known to belong 

 to the second chromosome (Bridges and Sturtevant, '14). A pair 

 of Fi wild-tj-pe flies inbred gave the results of table 3. 



Since cream only shows itself where eosin is already present, we 

 may disregard all the flies of culture 70 except those with eosin 

 eyes. Thes^e eosin flies are obviously in the ratio 2:1:1:0 

 which is expected if the cream gene is in the second chromosome, 

 though the flies a troeo few to prove the point. 



