Poiut Mutants — Detection and Effects 



207 



sives also carries /J7+, though this is not indi- 

 cated in the formula. 



Females of this stock do not produce 

 gametes containing X chromosome cross- 

 overs, since these are eliminated in the same 

 manner as in the Basc'^ heterozygote. 

 When females and males of this stock are 

 mated together, only two kinds of male 

 zygotes are produced. However, the half re- 

 ceiving the /-containing X die, while the other 

 half, which live, are genotypically exactly like 

 their father. The female zygotes are also of 

 two equally frequent types, the homozygotes 

 for IJl dying, the ones surviving being identi- 

 cal in genotype to their mother. This stock 

 is self-perpetuating because it contains a bal- 

 anced lethal system (see Chapter 20) in which 

 the females are permanent heterozygotes and 

 the males are of only one type. Note that 

 if nondisjunction produces an XO zygote, 

 this will die because Of the hemizygosity either 

 of IJl or /. There are three possible types of 

 nondisjunctional XXY females. One type 

 which is homozygous for X's carrying IJl, 

 lives, because of the />//+ on the Y, but it can- 

 not breed because females that have oc in 

 homozygous condition are sterile; the XXY 

 type homozygous for / dies. The third type 

 of XXY lives and breeds, but since it is het- 

 erozygous for the two kinds of X's, the 

 usefulness or continuity of the stock is not 

 impaired. 



Phenotypically, males are ocelliless and 

 bar-eyed, while females are slightly bar-eyed, 

 but otherwise wild-type. You can readily 

 see that mutations in the X of the male which 

 involve any of the fifteen normal loci aflect- 

 ing the eyes, body color, wings, or bristles, 

 for which the female has recessive alleles, 

 may be detected in their daughters. For ex- 

 ample, if a sperm carries a mutation from y^ 

 to j;, this will produce a yellow daughter when 

 it fertilizes an egg carrying the multiple reces- 

 sive X. This stock is, therefore, called 

 "Maxy" because it was designed for the 

 finding of "visible" mutations in the male's 



Ov o 



c 



4- 



J_ 



1 



OQ 



c 



. T 

 ^ 1 

 t 



FIGURE 24-2. The sex chromosomes of the males 

 and females used hi the Maxy technique. 



X at specific loci, including y. The Maxy 

 stock permits the detection of any mutation, 

 spontaneous or induced, involving the nor- 

 mal alleles of the fifteen recessives, provided 

 the mutant does not produce the normal 



