ITS ( II AIM IK 12 



I 2. is. In Drosophila each ol the genes for curly wings (Cy), plum eye color (/'///). 

 hairless ( // ) and dichaete wings ( /> ) are lethal when homozygous. A curly, 

 han less male mated to a plum, dichaete female produces 16 equally frequent 

 types ol sons and daughters. One curly, plum, hairless, dichaete F, son is 

 irradiated \uth \-ravs ami then crossed to a plum, dichaete female. Three F 2 

 sons phenotypically like the father, collected and mated separately with wild- 

 type females, produce the following males and females in the F 3 progeny. 



Phenotype Son I Son 2 Son 3 



C\ H 140 120 76 



( \ D 120 81 



Pm H 135 84 



Pm D 154 117 79 



Explain these results, using cytogenetic diagrams for all individuals mentioned. 



I2.1 l >. (a) Discuss the frequency of abortions in normal mothers who produce half- 

 translocational children with Down's syndrome. 



(b) Would you sometimes expect the occurrence of children with Down's 

 s\ndrome to he correlated with the father but not with his age? Explain. 



12.20. The Y chromosome is of different size in different phenotypically normal men. 

 On the other hand, a woman with a small X chromosome is phenotypically 

 defective. How can you explain the origin of such different Y and X chromo- 

 somes and the difference in the way they affect the two sexes? 



