LETHAL FACTORS IN DROSOPHILA 541 



The tests give 19 high to 22 low ratios which is the expecta- 

 tion for lethals, i.e., equahty is expected and is approximately 

 realized. 



Daughters from 2 : 1 cultures all of which were heterozygous 

 for white and half of which should be heterozygous for lethal 

 also, were again mated to white eyed males with the results in 

 table 7. 



There were 13 high to 13 low ratios shown by these daughters 

 indicating a lethal factor. On the basis of these data the locus 

 of the lethal is at 23.7. 



It is interesting to note that the lethal factor occurred in flies 

 that had been inbred a year and that none appeared in the stock 

 having been inbred only two months. Miss Rawls (Biol. Bull. 

 '13) found her lethal in a stock that had been inbred a year. 

 The lethals described by Quackenbush (Sc. '10) and Morgan 

 (Sc. '12; and Jour. Exp. Zool. '14) appeared in stocks that had 

 been inbred for some time. To test whether, in general, lethals 

 are more frequent in inbred stocks I mated 100 pairs from wild 

 stock caught at Falmouth, Mass., and 70 pairs from wild stock 

 caught at Harris, Minn. The results are shown in table 8. 



Tables 1 and 8 show that the counts made of offspring from 

 270 pairs of fresh wild stocks have no unusual ratios. 



THE SECOND LETHAL FACTOR 



On February 10 of 1914, sixty pairs from stock collected in 

 the summer of 1910 were mated. The results are shown in 

 table 9. 



The next to the last pair of the above table seemed to show 

 the presence of a lethal factor. Sixty virgin daughters from 

 this pair were mated to brothers. Nearly one-half of these 

 gave a ratio of twice as many females as males, as shown in 

 table 10. 



Virgin females from numbers 7, 8, 12, 23 and 59 were mated 

 to white eyed males. About one-third of the counts of the 

 next generation show the presence of a lethal factor (table 11). 

 Some of these lethal females were again mated to white eyed 



