32 GENETICS 



spring get from the mother a defective X (white), the 

 other half get from her a normal X (black). This is true 

 both for the daughters and the sons. But the daughters 

 get, also, a normal X from the red-eyed father. So they 

 all have normal red eyes. But the sons do not get an X 

 from the father; the only X they have is the one from the 

 mother. And those that get the normal X from the mother 

 have red eyes, while those that get the defective X have 

 white eyes. That is, there is now produced a family in which 

 all the daughters have red eyes, while half the sons have 

 red eyes, and the other half have white eyes. In a family 

 of two hundred, there are a hundred red-eyed daughters, 

 fifty red-eyed sons, and fifty white-eyed sons. The white 

 eyes are present in all cases where the defective chromo- 

 some from the grandfather Is the only kind present. 



There are many ways to test this, and it is always veri- 

 fied. For example, if we cross one of the red-eyed mothers 

 that has one defective X-chromosome, with a white-eyed 

 father, that also has the defective X-chromospme (fig- 

 ure 13), then we find that half the sons, and also half the 

 daughters, have white eyes, the other half red eyes. And 

 this is exactly the result given if the white eyes appear 

 only where the defective X is the only kind present. 



Since such characters appear in all males that have the 

 defective X (since they have no other), while in females 

 they appear only if both the X's are defective, they are 

 much commoner in males than in females, and are, hence, 

 often called sex-linked characters. But they appear in fe- 

 males as well as in males, provided that both the X's are 

 defective. 



Many Types of X-Chromo somes Exist. — Thus it is clear, 

 up to this point, that there are among the different indi- 

 viduals of the fruit-fly at least three kinds of X-chromo- 

 somes, with different effects. Some are normal, producing 

 normal red eyes. Others have become defective in such a 



