I9IS-] THE HEREDITARY MATERIAL. 149 



stance, the factor that determines the character for white eyes is 

 sex linked, as is also the factor that determines the character for 

 miniature wings. If we cross a female with white eyes and minia- 

 ture wings to a male with red eyes and long wings, the sons will 

 have white eyes and miniature wings. The explanation of this 

 result is found in the distribution of the chromosomes. The sons 

 get their single X chromosomes from their mother. Hence they 

 show the characters that this chromosome carried in the mother, 

 who had white eyes and miniature wings. The daughters, how- 

 ever, get one of their X chromosomes from their father through 

 his female producing sperm. This chromosome carried a factor 

 for red eyes and another for long wings, which factors dominate 

 those carried by the other X chromosome that the daughters get 

 from their mother, namely, the factors for white eyes and for 

 miniature wings. These relations are shown in Fig. 4. 



If these daughters and sons are bred to each other they produce 

 four kinds of individuals, viz., red long, white miniature, red minia- 

 ture, and white long. These are the four classes that Mendel's law 

 calls for, but they do not occur in the Mendelian proportion 

 (9:3: :3:i) when two pairs of factors, as here, are involved. 

 The reason for this is two-fold. In the first place the female alone 

 carries two X chromosomes. The male carries but one. Hence 

 there is an unequal distribution of the X chromosomes in the 

 spermatozoa, for, only half of them can get an X chromosome. 

 These are the female-producing spermatozoa. The result is, as has 

 been shown, that in the first generation the sons inherit their single 

 X chromosome from their mother and none of the dominant char- 

 acters of the father. Since in this case the sons carry no dominant 

 factor either in their X bearing (female producing), or in their 

 Y bearing (male producing sperm), the second generation here 

 reveals completely the composition of the egg cells that the F-^ 

 female carries. 



On Mendel's law of random assortment of two pairs of factors 

 w^e should expect the four classes that here appear in the second 

 generation to be equal in number. On the contrary we find that 

 two of them are twice as numerous as the other two. On inspec- 

 tion we see that the two larger classes are white miniature and red 



