OTHER METHODS OF SEX-DETERMINATION 221 



each attached to a member of the same pair of autosomes. 

 In the mature egg one such compound chromosome (there- 

 fore one X) is left in each egg. In the male, one X is pres- 

 ent, attached to the corresponding autosome, but the 

 other autosome has no attached X. After maturation half 

 of the sperm-cells will contain an X, half will be without 

 an X. The mechanism for sex-determination is obviously 

 here the same as in the XX-XO type. 



In the female, crossing-over might take place both be- 

 tween the two X-chromosomes and between the two at- 

 tached autosomes. But in the XO male the situation 

 would be different ; for in the male the X part of the com- 

 pound chromosome has no opposite, hence no crossing- 

 over is expected in that part. This would insure the 

 coherence of the sex-differentiating genes and of the sex- 

 mechanism ; but between the autosomal parts of the com- 

 pound chromosome an interchange might then take place 

 without affecting the sex-mechanism. The characters 

 whose genes lie in the X-component will show sex-linked 

 inheritance, i.e., the recessive character, will appear in 

 the sons. The recessive characters whose genes are in the 

 autosomal part will not appear in the sons. However, the 

 character whose genes are in the autosomal part will 

 show partial linkage to sex and to the characters whose 

 genes lie in the X-component.^ 



In the imaginary example just given, the autosome 

 without an attached X, that is, the mate of the compound 

 chromosome with an X in the male, will appear to corre- 

 spond to the Y-chromosome of the ordinary XX-XY type 



1 According to McClung the X-chromosome in the male of Hesperotettix 

 is not constantly attached to the same autosome, although in a given indi- 

 vidual its attachment is constant. In other individuals it may be free. Were 

 sex-linked characters known in such a type, their inheritance might be 

 complicated by this inconstant relation of the X-chromosome to the auto- 

 somes. 



