MUTATION 



261 



tliey will throw off a small predictable munber of a 

 "mutant" type— a type that we can introduce into the 

 stock for the express purpose of recovering it by such an 

 apparent mutation process. 



For example, dichete is a third chromosome dominant 

 wing-and-bristle character and, like beaded, a recessive 

 lethal. Sturtevant bred flies with the gene for dichete 

 in one of the third chromosomes and with a gene for the 

 recessive eye-color, peach, in the other for several genera- 

 tion crossover eggs _ 



(95^ of total) SP®"^"" 



D'- 

 N 



H 



1» 



P 



D'. 



■P 



TSm 



Hlap 



1X1 



i:^ 



KlaP 



Fia. 113. — Diagram illustrating how in the presence of a dominant factor, dichete, and 

 a lethal in its homologous chromosome at about the same level, together with another 

 factor, peach-colored eyes (p), gives the result shown in the squares. No peach appears 

 in the offspring except where crossing over takes place as shown in the next diagram. 



tions. A lethal appeared by mutation in the peaoh-bearing 

 chromosome very near the level of the dichete gene in 

 the opposite chromosome. 



The order of these genes is shown in Fig. 113. This is 

 then a balanced lethal stock that throws only dichete 

 flies,^ except for a small percentage of dichete peach flies 

 due to crossing over. The result for the non-crossover 

 classes is shown in the square to the right. Only two 

 of the four classes come through : the two that die are the 



' Very rarely a crossover not-dichete fly will appesu". 



