76 AN INTRODUCTION TO MODERN GENETICS 



XX-XO chromosome mechanism. No cases are known of the opposite 

 phenomenon, the disappearance of the Xy which would leave sex 

 determination dependent on the presence of a single supernumerary 

 chromosome in one sex {OO-OY mechanism). 



Either the X or the Y may become compound either by a reciprocal 

 translocation with an autosome, or by fragmentation, which may per- 

 haps always be a result of translocation (p. 95). A translocation may 

 produce ring formation or chain formation of chromosomes in meiosis 

 analogous to the phenomena found in Oenothera. The XX-XY 

 mechanism may sometimes be secondarily produced from an XX-XO 

 type by fusion of the X's with an autosomal pair, giving an equal 

 A -\- X pair in one sex and an unequal A + X^A pair in the other. 



Fig. 31. Some Types of Sex Chromosomes. — The chromosomes are shown in 

 side view of meiotic metaphase, the Y being above. A equal X and Y (e.g. many 

 Hemiptera), 6 and C smaller Y (e.g. rat), D larger Y (e.g. Drosophila), £ com- 

 pound Y (e.g. Humulus), F compound X (e.g. Tenodera), G compound X (Blaps), 

 H fusion of X to autosome (e.g. Mermiria). 



(After Wilson, Darlington, etc.) 



This State of affairs can sometimes be recognized by the occurrence of 

 precocious condensation (heteropycnosis) of the X-chromosome part in 

 the prophase of meiosis. In other cases the evidence is more indirect; 

 e.g. Drosophila melanogaster has a rod-shaped X and F-shaped auto- 

 somes, D. willistoni has a F-shaped X (and one pair of rod-shaped and 

 one of F-shaped autosomes) and part of the X is homologous, as is 

 shown by comparison of the linkage maps with one of the autosomes 

 of melanogaster} 



2. Cytological Behaviour of the Sex Chromosomes 



The peculiarities in the cytological behaviour of the sex chromo- 

 somes can be regarded as consequences of, or adaptations to, the fact 

 that the members of the XY pair are unlike and must be segregated by 

 some mechanism which does not allow their differences to be aimulled 

 by crossing-over. The simplest case is of course that of the XX-XO 

 mechanism, where in meiosis the isolated chromosome of the hetero- 

 gametic sex either (i) passes undivided to one pole in the first division, 

 ^ Cf. Morgan, Bridges and Sturtevant 1925. 



