Mutations and Evolution. 185 
father bears a dominant sex-linked character. About 50 such 
sex-linked mutations are now known in Drosophila. 
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 or zero 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 
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 + 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 + 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 
