SEX-LINKAGE 



IPROSOPHILA 



xx~9 



Y ^^Sr^:^'^'''''' ^ 



/Reciprocal C/iiasma/a. 



CO. per ccnfT— O 5 9 14 28 55 



Re5ucfionaI 



Se[)arof1on:^ EqualTonol 



X 



Y 



Tfc=^ 



^ yI^ 



Fig. 13. — Crossing-over between the pairing segments of X and Y in heterogametic 

 (male) sex oi Drosophila and man showing the structure of the bivalent at pachytene 

 and first metaphase of meiosis. The pachytene pairing gives a metaphase orien- 

 tation and consequent separation at the first anaphase which depends on the position 

 of crossing-over and chiasma formation. In Drosophila the genes bb, bobbed, and B, 

 Bar, etc., and in man the genes c.b., colour blindness, ha, haemophilia, iV, ichthyosis, 

 etc., are marked. The crossing-over distances in man are from Haldane. Black, 

 pairing segment with partial sex linkage through crossing-over between X and Y 

 (as well as between X's in females). Hatched, differential segment with complete 

 sex linkage in X and with crossmg-over between X's in females. White, differential 

 segment with complete sex linkage in Y and no crossing-over whatever. 



53 



