SEX-LINKED INHERITANCE 



231 



along the lines of Mendelian segregation as is the case of sweet 

 peas. The sex chromosome (Fig. 101) may be represented by 

 X, the black X representing red eyes, the open X white eyes, 

 and O in males, no X. After maturation of the original white- 

 eyed male, one type of spermatozoa has the open X, the other 

 type has no X as in the case of Pro tenor (Fig. 98). Fertilization 

 affords an equal chance for both types of spermatozoa. The egg 



xx 



XXXI 



FIG. 101. Sex -linked inheritance of white and red eyes in Drosophila. 

 Parents, white-eyed o* and red-eyed 9 ; Fi red-eyed <? and 9 ; F 2 , red-eyed 9 , 

 red-eyed o* and white-eyed c? . To right of flies the history of the sex chromo- 

 somes XX is shown. The black X carries the factor for red eyes, the open X 

 the factor for white eyes, O stands for no X. (From Morgan.) 



cells contain one black X. The Fi generation will all contain a 

 black X which is dominant over the open X, and of course over 

 no X. When these are interbred, two black X 7 s, a black X 

 with an open X, a black X with no X, and an open X with no X 

 may result, and the proportions are three red to one white. 



In this experiment, only males appeared with white eyes, a 

 fact which might be interpreted as sex-limited inheritance, males 

 only having white eyes. This, however, is not true, for white- 

 eyed females may be produced by mating the above red-eyed 



