Detection of Visible Mutations 249 



tached-X females have the two attached X chromosomes and a 

 Y chromosome. They are females because of the two X's. When 

 such a female is crossed with a normal male, four types of indi- 

 viduals are theoretically possible. If the two attached-X's unite 

 with the Y chromosome from the male, an attached-X female is 

 produced; and if the Y chromosome from the attached-X female 

 parent unites with the X chromosome of the male parent, a 

 normal male is produced. When, however, the two attached 

 X chromosomes unite with the X chromosome from the male, an 

 individual is produced which has three X chromosomes. It is 

 known as a "super female," but it frequently dies at a very early 

 stage of development and is never fertile. The fourth theoretical 

 type results from the union of the Y chromosome of the at- 

 tached-X female with the Y chromosome of the male. These 

 individuals possess no X chromosomes and therefore do not have 

 one complete genome; tliey die at an early stage. Therefore, a 

 cross between an attached-X female and a normal male results 

 in an attached-X female and a normal male plus sterile and 

 inviable types. The interesting feature of this cross is that the 

 normal male offspring receive their X chromosome from their 

 male parent instead of from their female parent as is usual. 



In this method, stocks are prepared in which known recessive 

 genes are present. The recessives are different in the attached-X 

 female stock from those in the normal male stock, and, in this 

 way, the actual parentage of the individual X chromosomes is 

 known. The normal males used as the male parents are irradi- 

 ated and are then mated to females from the attached-X stock. 

 If a visible mutation was produced in the X chromosome of the 

 male parent, it would appear in the male offspring of a cross 

 between that male parent and an attached-X female because the 

 male offspring have all received their X chromosome from their 

 father and their Y chromosome from their mother. If normal 

 female stocks were used, a recessive mutation produced in the 

 X chromosome of the male parent would become lost in the next 

 generation because this X chromosome would be transmitted to 

 the female offspring and the recessive mutation would be "cov- 

 ered up" by the dominant allele in the other X chromosome. 

 The attached-X stock is therefore a very useful tool for detect- 

 ing these recessive mutations that have a visible effect on the 

 phenotype. 



