258 



PHYSICAL BASIS OF HEREDITY 



and sperm are indicated in the figure by the arrow-scheme 

 below, which gives the combinations (classes) included in 

 the four squares. The double dominant BB is the class 

 that does not come through. The result is two beaded 

 (heterozygous) to one normal fly. 



The beaded stock remained in this condition for a long 

 time ; although selected in every generation for beaded, it 

 did not improve, but continued to throw 33 per cent, of 

 normal flies. Then it changed and bred nearly true. 



Eggs 



Sperm 



B-- 



t 



N 



B"- --N 



5 ^^ 



ixt 



B N. 



Beaded : 1 Normal, 



Fig. 110. — Diagram showing the relation of the chromosomes (represented by the 

 vertical rods) in a cross of "beaded" by "beaded." Flies homozygous for beaded die as 

 indicated by the cross-hatched square. 



The change must have been due to the appearance of 

 another lethal factor (now called lethal three, here Zi) in 

 Fig. 111). Such a gene was found in the race when 

 studied later by Muller. 



The lethal gene that appeared in the beaded stock was 

 also in the third chromosome, and in the chromosome that 

 is the mate of the one carrying the gene for beaded, i.e., 

 in the normal third chromosome of the beaded stock. The 

 lethal gene lies so near to the level of the beaded-normal 



