The Cellular Basis 191 



(i.e. comes from either parent) all the daughters of a color blind 

 father and normal mother will appear normal although carrying 

 one determiner for color blindness, while all the sons will be nor- 

 mal because they carry no determiner for color blindness. But 

 these daughters transmit to one-half of their children the single 

 determiner for color blindness and if any of those receiving this 

 determiner are males they will be color blind. Consequently we 

 have the curious phenomenon of simplex color blindness appear- 

 ing only in males and being transmitted to them only through ap- 

 parently normal females. 



On the other hand if a female is color blind she has inherited it 

 from both father and mother, i.e., the character in her is duplex, 

 and in all of her children by a normal male the character will be 

 simplex; accordingly all of her sons will be color blind and all of 

 her daughters will be normal, though carrying the simplex deter- 

 miner for color blindness. 



Sex-linked Lethal Factors. One of the most interesting cases 

 of linkage is found where early death is linked with sex. In 

 Drosophila a considerable number of lethal mutant factors have 

 been demonstrated in the X chromosome and all individuals in 

 which such a factor is not balanced by a normal allelomorph die 

 early. All males that receive such a lethal die, since there is only 

 one X chromosome in the male ; all homozygous females that 

 have the factor in both X chromosomes die, while only those sur- 

 vive that are heterozygous for this factor. Such a heterozy- 

 gous female produces in equal numbers eggs with and without 

 the lethal factor and if she is bred to a normal male all of the 

 daughters are viable though half of them carry the lethal factor 

 in one of the X chromosomes, but all of the males that receive 

 the lethal factor are non-viable since the male has only one X 

 chromosome, while all the males that survive lack this factor 

 altogether. Thus the sex ratio in this case is 2 females to i male. 

 Other lethal factors have occurred in other chromosomes of 



