NEW DATA. 



65 



is 22.2. A correction of 0.4 unit should be added for double crossing- 

 over, indicating that the locus is 22.6 units from white, or at 23.7. 



When the work on lethal sa had been continued for 3 months, the 

 second lethal, lethal sb^ was found (April 191 3) to be present in a female 

 which was already heterozygous for lethal sa. It is probable that this 

 second lethal arose as a mutation in the father, and that a sperm whose 

 X carried lethal sb fertilized an egg whose X carried lethal sa. As in 

 the cases of lethals i and la and lethals 3 and 3<3, this lethal, lethal 

 j^, was discovered from the fact that only a very few sons were produced, 

 there being 82 daughters and only 3 sons. If, as in the other cases, the 

 number of daughters is taken as the number of non-cross-overs and 

 twice the number of sons as the cross-overs, it is found that the two 

 lethals are about 7 units apart. Since the two lethals were in different 

 X chromosomes, all the daughters should receive one or the other lethal, 

 except in those few cases in which crossing over had taken place. Of 

 the daughters 19 were tested and every one was found to carry a lethal. 

 Again, if the cross-over values of the lethals with some other character, 

 such as white eyes, be found and plotted, the curve should show two 

 modes corresponding to the two lethals. This test was applied, but 

 the curve failed to show two modes clearly,^ the two lethals being too 

 close together to be differentiated by the small number of determina- 

 tions that were made. It seems probable that lethal sa and lethal sb 

 are about 5 units apart. 



The position of lethal sb was accurately found by continuing the 

 determinations with a white lethal cross-over. A white female was 

 found which had only one of the two lethals and the linkage of this 

 lethal with eosin and miniature was found as follows : A female carrying 

 white and lethal in one chromosom.e and no mutant factor in the homol- 

 ogous chromosome was bred to an eosin miniature male. The white 

 eosin daughters carried lethal, and their sons show the amount of 

 crossing-over between white and lethal (15.6), between lethal and mini- 

 ature (19.9), and between white and miniature (32.9). The data on 

 which these calculations are based are given in table 48. 



Table 48. — Data on the linkage of white, lethal sb, and miniature, 



from Stark, 1915. 



^The curve published by Miss Stark included by mistake 6 cultures from the succeeding gen- 

 erations, and these coming from only one of the lethals (lethal sb) increase its mode so that the 

 mode of the other lethal (lethal sa) becomes submerged. If these cultures are taken out the 

 curve shows two modes more clearly. 



