82 T. H. MORGAN 



THE FIRST LETHAL FACTOR 



The origin of this stock has already been described (Rawls/ 

 Morgan,-). The fir^ part of the data that follow includes those 

 already given in my former paper. At the time when that 

 paper was written the 'returns' were still coming in. Most of the 

 counts here given for the first lethal were made by Mr. J. S. 

 Dexter who was assisting me at the time. 



Females from the lethal stock sent to me by Miss Rawls were 

 mated to white-eyed males in pairs. Some pairs gave 1 :1 ratios, 

 i.e., equality in the sexes; and others gave 2:1 ratios, i.e., two 

 daughters to one son. Those mothers that gave as many sons 

 as daughters (e.g., pairs OM and K) were not heterozygous for 

 lethal, and all their offspring should continue normal. Those 

 mothers (e.g., pair L) which gave only half as many sons as 

 daughters should be heterozygous for the lethal factor; and the 

 missing sons should be those that received this lethal bearing 

 sex-chromosome from their mother. 



To bring out this difference clearly, daughters (heterozygous 

 for white) from the normal lines OM and K were mated to white 

 males. That they did not carry the lethal factor is shown in 

 table 1 by the production of a 1:1:1:1 proportion, which is that 

 normally expected from a cross between a white male and a red 

 female heterozygous for white. 



In contrast to the results of table 1 are those from the mat- 

 ing of daughters (heterozygous for white) from the pair L, which 

 gave a 2:1 ratio, to white males. The results show that while 

 half the daughters of pair L gave a normal sex-ratio (table 2), 

 the other half gave the lethal sex-ratio of 2:1 (table 3). 



The explanation of the fact that half of the daughters of L 

 gave lethal sex ratios, and half gave normal ratios is that their 

 mother was heterozygous for lethal. Half of her gametes were 

 lethal and half non-lethal, so that of her daughters^ half again 

 should be heterozygous for lethal and half free from lethal. 



1 Biological Bulletin, vol. 24, 1913. 



2 Science, vol. 36, pages 718-20, 1912. 



