MATERIALS OF HEREDITY 29 



Other X-chromosomes, in other Individuals of Drosophila, 

 are found to produce still other effects. Some are defective 

 or abnormal, but instead of producing bar-eyes, they pro- 

 duce other abnormalities. And most of them behave in a 

 way that differs from the behavior of bar-eye. An example 

 will make clear this behavior. 



In certain cases X-chromosomes produce eyes that are 

 white instead of red. Normally in the fruit-fly the eyes are 

 red. But sometimes there are found individuals whose eyes 

 are white. When these are mated together, they produce 

 offspring all of whose eyes are white. So the white eyes are 

 hereditary. 



But what happens if one mates together red-eyed and 

 white-eyed parents? What will be the color of the eyes in 

 the offspring? Mate first a white-eyed mother with a red- 

 eyed father (figure 11). In this case the figure represents 

 the mother's X-chromosome in white (since these are the 

 abnormal individuals), while the father's are shown in 

 black. Thus the descendants of the two kinds can be traced 

 in later generations. 



The sons get an X-chromosome from the mother only. 

 And they all have white eyes, like the mother. The daugh- 

 ters get one X from the mother, one from the father. And 

 they all have red eyes, like the father. 



So the white eyes go with the mother's X-chromosome. 

 But if one of the father's X-chromosomes is likewise pres- 

 ent, then the white eyes are not produced; such daughters 

 have red eyes. So the red eyes are dominant. The white 

 eyes, since they do not appear if a normal X-chromosome 

 from the father is present along with the abnormal one 

 from the mother, are said to be recessive. White eyes ap- 

 pear only in individuals in which the abnormal X-chromo- 

 somes are the only kind of X present. 



All this can be tested In a great number of ways. Mate 

 for example one of these daughters, having the two kinds 



