NEW DATA. 79 



LETHAL SC. 



The third of the lethals which Miss Stark found (Stark, 1915) while 

 she was testing the relative frequency of occurrence of lethals in fresh 

 and inbred wild stocks arose in April 1914 in stock caught in 1 9 10. 

 Females heterozygous for this lethal, lethal sc, were mated to white 

 males and the daughters were back-crossed to white males. Half of 

 the daughters gave lethal sex-ratio, and these gave 1,405 cross-overs 

 in a total of 3,053 males, from which the amount of crossing-over 

 between white and lethal sc has been calculated as 46 per cent. 



By reference to table 65 it is seen that white and bar normally give 

 only about 44 per cent of crossing-over in a two-locus experiment;^ 

 lethal sc then is expected to be situated at least as far to the right as bar. 

 Females heterozygous for lethal sc were therefore crossed to bar males, 

 and their daughters were tested. The lethal-bearing daughters gave 

 144 cross-overs in a total of 1,734 males, that is, bar and lethal sc gave 

 8.3 per cent of crossing-over. Lethal sc therefore lies 8.3 units beyond 

 bar or at about 66.5. The cross-over value sable lethal sc was found 

 to be 23.5 (387 cross-overs in a total of 1,641 males) which places the 

 lethal at 43+23.5, or at 66.5. We know from other data that there 

 is enough double crossing-over in the distance which gives an experi- 

 mental value of 23.5 per cent, so that the true distance is a half unit 

 longer or the locus at 67.0 is indicated by the 1,641 males of the sable 

 lethal experiment. In a distance so short that the experimental value 

 is only 8.3 per cent there is, as far as we have been able to determine, 

 no double crossing-over at all, or at most an amount that is entirely 

 negligible, so that a locus at 57+8.3 or 65.3 is indicated by the 1,734 

 males of the bar lethal experiment. To get the value indicated by the 

 total data the cases may be weighted, that is, the value 65.3 may be 

 multiplied by 1,734, ^"^ ^7-° may be multiplied by 1,641. The sum 

 of these two numbers divided by the sum of 1,734 ^^^ i>64i gives 66.2 

 as the locus indicated by all the data available. This method has 

 been used in every case where more than one experiment furnishes data 

 upon the location of a factor. In constructing the map given in 

 diagram I rather complex balancings were necessary. 



LETHAL SD. 



The fourth lethal which Miss Stark found (May 1914) in the inbred 

 stocks of Drosophila has not been located by means of linkage experi- 

 ments. It is interesting in that the males which receive the lethal 

 factor sometimes live long enough to hatch. These males are ex- 

 tremely feeble and never live more than two days. There is, as far 

 as can be seen, no anatomical defect to which their extreme feebleness 

 and early death can be attributed. 



