694 General mid Applied Biology 



one P. White flowers are represented by at least one C and pp, by cc 

 and at least one P, and by ccpp. This may be shown as follows: 



When the Fi are crossed, the F2 ratio is: 



F2 9 purple (at least one C and at least one P) 



3 white (at least one C and pp) 

 3 white (cc and at least one P) 

 1 white (cc and pp) 



This phenotype ratio is 9 purple:? white. 



9. Lethel Genes. — When lethal genes (le' thai) (L. letum, death) are 

 present they may kill the organism containing them or at least prevent 

 the individual from attaining normal maturity (Fig. 341), Lethal genes 

 are known in mice, fruit flies, human beings, in certain plants, etc. In 



GAMETE 



DIES 



Fig. 341. — Inheritance of lethal, sex-linked factor in the fruit fly (Drosophila 

 sp.) and its effect on the sex ratio. /, The lethal factor; L, its normal, nonlethal 

 allele; P, parents (squares are males; circle are females); Fi, the first filial 

 generation in which one-half of the males fail to develop, giving the sex ratio 

 of two females to one male. Note that both parents (P) are normal but that the 

 female carries the lethal factor which she later contributes to one-half of her sons, 

 who consequently never develop. 



