96 



CHAPTER 7 



quencj from w to w or the reverse is 

 several orders of magnitude lower than the 

 observed frequency of exceptional Hies. 



Since the exceptional Fi females are white- 

 eyed, each must cam \ V | Figure 7-7B). 

 The onlj source of X's containing w is the 

 mother. Accordingly, the father must fail 

 to contribute his X"' chromosome to an 

 exceptional daughter. Each exceptional 

 dull-red-eyed son must carry X"", which 

 could be contributed only by the father. 



In order to understand how this excep- 

 tional situation may come about, let us 

 examine the normal consequence of meiosis 

 in the Drosophila female as regards the sex 

 chromosomes. Normally, the two X's syn- 

 apse and form a tetrad, and due to segrega- 

 tion four nuclei are produced at the end of 

 meiosis, each containing one X (Figure 

 7-8 A ) . One of the four nuclei becomes the 

 gametic (egg) nucleus; the other three are 

 discarded (in polar bodies). 



Suppose, however, segregation of the four 

 strands in the X chromosome tetrad occa- 

 sionally occurs improperly in either of two 

 ways: 



1. At anaphase I, instead of one dyad 

 going to each pole, both dyads go to the 

 same pole (Figure 7-8B). The daughter 

 nucleus containing no X dyad undergoes the 

 second meiotic division to produce two 

 nuclei, neither one having an X. The other 

 daughter nucleus, containing two dyads, pro- 

 ceeds through the second division, during 

 which the two members of each dyad sepa- 

 rate and go to opposite poles at anaphase II. 

 The result is two daughter nuclei each con- 

 taining two X's, one from each dyad. 

 Therefore, at the end of meiosis, the failure 

 of dyads to disjoin at anaphase I will result 

 ultimately in four nuclei, two with no X and 

 two with two X's. As a consequence the 

 nucleus which becomes the gametic nucleus 

 has a 50% chance of carrying no X and a 

 50% chance of carrying two X's. 



2. Alternatively (see Figure 7-8C), 



METAPHASE TELOPHASE 



figure 7-8. Consequences of normal segrega- 

 tion of X chromosomes (A) and of its failure 

 to occur (B and C) . 



anaphase I is normal, and at telophase I 

 two daughter nuclei are formed each con- 

 taining one X dyad. The second meiotic 

 division occurs normally in one of the daugh- 

 ter nuclei, producing two telophase II nuclei, 

 each of which contains one X. In the other 

 daughter nucleus, however, the members of 

 the X dyad fail to separate at anaphase II 

 and go instead into the same telophase II 

 nucleus. This failure of monads to disjoin 

 at anaphase II produces two nuclei, one con- 

 taining no X and the other containing two 

 X's. Consequently, the gametic nucleus has 

 a 25% chance of carrying no X, a 25% 

 chance of carrying two X's, and a 50% 

 chance of carrying one X. 



The occasional failure of normal separa- 

 tion of chromatids at either the first or the 

 second meiotic division would result in the 

 occasional production of eggs containing 

 either no X or two X's. Such a failure of 

 the members of a pair of chromosomes to 

 segregate is also referred to as nondisjunc- 

 tion of chromosomes. According to the hy- 

 pothesis that the X chromosome carries an 

 allele for w, chromosomal nondisjunction 

 can provide the mechanism by which a pair 

 of genes fails to segregate, with the result 

 that after meiosis, eggs are sometimes pro- 

 duced containing two members or no mem- 



