276 GENERAL ZOOLOGY 



whether, in addition to the autosomes, 1 or 2 X chromosomes are 

 present. Under normal conditions, if only 1 X chromosome is 

 present, there is also present a Y chromosome. Such an egg 

 develops into a male. If 2 X chromosomes are present, the 

 presence of a Y chromosome is excluded, and such an egg develops 

 into a female. 



Sex-linked Inheritance. — The results obtained by Mendel in 

 his experiments with peas were the same in the F\ and F 2 genera- 

 tions regardless of the parental origin of the allelomorphic 

 characters. In the yellow-green cross, for example, yellow 

 gene may be supplied by the egg and green by the pollen, or 

 vice versa — the result is the same. The genes for these charac- 

 ters are common to both sexes and do not seem to be related in 

 any way with the genes for sex. A sex-linked character, on the 

 other hand, is one whose genes are linked in some way with the 

 genes for sex. The eye color of wild Drosophila is red. Flies 

 with white eyes appeared in cultures grown in the laboratory. 

 White eye color is an example of what is known as a mutation, 

 i.e., a sudden and spontaneous variation, the cause of which is 

 unknown. That the cause of the mutation is due to a change in 

 the chromosomes or genes is indicated by the fact that the charac- 

 ter is inherited. White-eyed flies breed white-eyed flies. The 

 gene for white eye seems to be the result of a change in the gene 

 for red eye, which is its allelomorph. If a normal red-eyed 

 female of Drosophila is bred with a white-eyed male, the Fi 

 generation is red-eyed, males and females alike. If males and 

 females of the F\ are bred, there are produced in the F 2 red-eyed 

 females, red- and white-eyed males, but no white-eyed females. 

 If the reciprocal cross is made by starting with a white-eyed 

 female and a red-eyed male, the F\ consists of red-eyed females 

 and white-eyed males. These produce in the F 2 red- and white- 

 eyed males and females in equal numbers. The fact that the 

 results differ, depending upon the parental origin of the charac- 

 ters, suggests that there is some connection between the genes 

 for sex and those of eye color. 



A simple explanation for the results may be reached by making 

 the following assumptions: (1) that red eye is dominant to its 

 allelomorph, white eye; (2) that the genes for both are located 

 in the X chromosome; (3) that 2 X chromosomes produce a 

 female ; and (4) that the Y chromosome does not carry a gene for 



