124 GENETICS 



uals. Through non-disjunction, male individuals are pro- 

 duced that contain no Y (see Chapter III). 



(b) But the presence of Y is necessary for the fertility 

 of males. 



(c) Y may exist in females, as a result of accidental 

 non-disjunction; it does not affect their sex. 



(d) During many years of intensive study no charac- 

 teristics were found that follow the Y-chromosome. 



(e) But recently it has been shown that the presence of 

 Y prevents the appearance of a certain recessive character 

 ("bobbed") that is due to a modified X-chromosome. This 

 modified X results in the production of shortened bristles 

 on the body of the fly. But this peculiarity does not oc- 

 cur if a Y is present, so that Y has the dominant normal 

 allele for "bobbed." Indications of certain other ill-de- 

 fined functions of the Y-chromosome have of late come to 

 view. 



(f) No diverse modifications or defects such as are 

 known in great number for the X-chromosome are known 

 in the Y-chromosome. 



(g) Thus in Drosophila the Y-chromosome appears to 

 have little function; it seems nearly but not quite inert. 



Active Y-Chromosomes in Certain Organisms; Relation 

 to Male Secondary Sex Characters: In certain organisms 

 the Y-chromosome is more active and varied in its effects. 

 In such cases it shows in different individuals diverse modi- 

 fications, comparable to the diverse types of the X-chromo- 

 some in Drosophila, and these produce diverse inherited 

 characteristics that appear in males only. 



This matter is closely bound up with the question of 

 what produces the diversity of secondary sex characters. 

 In many animals there are special characteristics which, 

 like the Y-chromosome, are found only in males. These 

 are the male secondary sex characters: horns, mane, beard, 



