A. PHENOTYPES 



R White 



CHAPTER 12 



Red 



F, TYPICAL 



white 



L S 



Idull 



</c/ 



lull (5 



EXCEPTIONAL <" 



di 

 red 



|_white Q 



The following reasoning can be used to 

 attempt to explain these results. Since the 

 exceptional females are white-eyed they must 

 contain X^'X"" (Figure 12-8B). The only 

 source for X's containing w was their mother. 

 Accordingly, their father must have failed to 

 contribute to them his X""^ chromosome. 

 The exceptional dull red-eyed sons must 

 carry X"** which could have been received only 

 from their father. In order to understand 

 how this exceptional situation may come 

 about, let us recall first the normal conse- 

 quence of meiosis with regard to the sex 

 chromosomes in the Drosophila female. Nor- 



B. GENOTYPES 



WW w 



X X X X Y 



w 

 X Y 



w w 

 X X 



[w wl 

 X xj 



FIGURE 12-8. Nonmutant exceptions in crosses 

 involving eye color in Drosophila. 



mally, the two X's synapse and form a tetrad, 

 and at the end of meiosis, because of segrega- 

 tion, four nuclei are produced each containing 

 one X (Figure 12-9 A). Then one of the four 

 nuclei becomes the gametic (egg) nucleus 

 while the other three are discarded (in polar 

 bodies). 



Suppose, however, that occasionally sepa- 

 ration of X chromosomal material in the 

 tetrad fails to occur properly in either one of 

 the two following ways. At anaphase I, in- 

 stead of one dyad going to each pole, both 

 dyads go to the same pole (Figure 12-9B). 

 The nucleus containing no X dyad then pro- 

 ceeds through the second meiotic division 

 and gives rise to two nuclei, neither of which 

 contains an X. The other nucleus, containing 

 two dyads, proceeds through the second 

 division, during which the two members of 



