OF MUTANT CHARACTERS. 289 



and lethal is the reverse of that from lethal and curved, since black and 

 the lethal are in different homologues. The equations in this case are 



from which the black lethal value is 20.3. 



Culture 2860 gave x = 5, n = 20 and a lethal curved cross-over value 

 of 20. 



The foregoing cultures have given in the case of black and lethal a 

 total value of n of 144.7 and of x of 21.7, so that the cross-over value 

 is 13.0. Likewise for lethal-curved, n = 227.1, a: = 21. 6, and the cross- 

 over value is 8.7. 



STOCK OF LETHAL lla. 



It was foreseen that stock of this lethal, which was called lethal 

 I la (the "II" designating the chromosome, and the "a" denoting the 

 first of this series), could not be run advantageously by means of cul- 

 tures such as were made from the wild-type flies of the original F 2 . 



A stock could be maintained by taking advantage of the fact of no 

 crossing-over in the male, if males carrying the lethal in one second 

 chromosome and some recessive in the other were back-crossed in each 

 generation to females homozygous for this same recessive. An effi- 

 cient scheme for obtaining such a stock was devised as follows: The 

 original F 2 flies appeared only in the three classes: wild-type, black, 

 and curved. The constitution of any given wild-type fly could not 

 be told by inspection, but the case was different with the blacks and 

 curveds. Their father had carried black, lethal, and curved all in the 

 same second chromosome, and since there is no crossing-over in a male, 

 every sperm which carried one of these genes carried all. Therefore 

 every black fly in the F 2 should have the black lethal curved second 

 chromosome of the father. 



The maternal second chromosome is in this case a cross-over chromo- 

 some, carrying black not-lethal, and not-curved. If such a male 



"- ), were crossed to a curved female, the offspring should be of 

 6 I c / 



two kinds wild-type ( ^^ ) and curved ( - ), and every one of 

 \++c/ \++c/ 



these curved flies should carry the lethal in the manner required. In- 

 stead of simple curved, a black purple curved plexus speck (TT ) 

 female was used in this out-cross the object being to combine the TT 

 stock with the ln a stock so that no separate TT or l lla stocks need be 

 maintained. Three such out-crosses of F 2 black males to TT females 

 were made. Two of these gave only black offspring (culture 2865, 155 

 blacks; culture 2866, 182 blacks), and no black curved, which showed 

 that the males had been cross-overs belonging to the F 3 generation. To 

 check up this analysis, five F 2 cultures were raised from one of them 



