124. THEORY OF EVOLUTION 



When these two Fi flies are inbred (middle 

 row) the following combinations are expected. 

 Half the eggs will contain each a white pro- 

 ducing X chromosome and half red producing. 

 The female-producing sperms will each con- 

 tain a white X and the male-producing sperms 

 will each contain an indifferent Y chromosome. 

 Chance meetings of egg and sperm will give the 

 four F2 classes (bottom row) . These consist 

 of white eyed and red eyed females and white 

 eyed and red eyed males. The ratio here is 

 1 : 1 and not three to one (3:1) as in other 

 Mendelian cases. But Mendel's law of segre- 

 gation is not transgressed, as the preceding 

 analysis has shown; for, the chromosomes have 

 followed strictly the course laid down on Men- 

 del's principle for the distribution of factors. 

 The peculiar result in this case is due to the 

 fact that the Fi male gets his single factor for 

 eye color from his mother only and it is linked 

 to or contained in a body (the X chromosome) 

 that is involved in producing the females, while 

 the mate of this body — the Y chromosome — is 

 indifferent with regard to these factors, yet 

 active as a mate to X in synapsis. 



