Mutations and Evolution. 



father bears a dominant sex-linked character. About 50 such 

 sex-linked mutations are now known in Drosophiln. 



But occasional exceptions occur, in which a female produces a 

 daughter like herself or a son like the father. These primary 

 exceptions by an ordinary XX female were found to be the result of 

 an egg in maturation either retaining both X chromosomes or 

 allowing both to pass out into the polar body. Such an egg will be 

 XX orzero as regards its sex chromosomes. If such eggs are fertilized 

 by ordinary sperm from a red-eyed male, the result will be as 



vermilion red 



cr 



(I) Dies 



(2) Vermilion ? 

 exception 



(3) Red <J (4) Dies 

 exception 

 (sterile) 



Fig. 1. Diagram of primary non-disjunction. Modified after Bridges. 

 shown in the diagram (Fig. 1). Four new types will result: 

 (1) The XX egg -f X sperm = XXX females which die. (2) The 

 XX egg + Y sperm = XXY female having vermilion eyes and 

 hence an exception to the criss-cross rule. (3) The O egg -f X 

 sperm = XO male which is sterile. It has red-eyes and is a 

 patroclinous exception to the criss-cross rule. (4) The O egg 

 + Y sperm = OY zygote which is not viable. Hence only 



