OPERATION OF THE GENETIC SYSTEM 57 



the gametes (ova) are alike as to their chromosomes — 

 each containing n autosomes plus X. Sex is determined by 

 the type of sperm that enters an ovum. A sperm of the 

 constitution (autosomes ~j- X) uniting with an ovum (auto- 

 somes + X) gives a female (2 sets of autosomes -\- XX). 

 A sperm of the constitution (autosomes + Y) uniting with 

 an ovum (autosomes -f X) gives a male (2 sets of auto- 

 somes + XY). (In all such cases the Y may be lacking.) 

 To Group I belong many groups of organisms, including 

 man. The following groups of animals are among* those that 

 show the relations of Group I : echinoderms, nematodes, 



c<«wn»»ccciw*» 



Figure 19. The twenty-four pairs of chromosomes in man, ar- 

 ranged in order of size, after Evans and Swezy. The small unequal 

 pair at the right end of the lower row are X and Y. 



mollusks, most insects, arachnids, myriapods, fish, mam- 

 mals. 



Four main subtypes may be distinguished in Group I, dif- 

 fering in the condition found in the males. These are: 



Subtype I. Male has X but no Y (figure 3). 



Subtype 2. Male has a large X and a small Y (figure 6). 

 To this group belongs man (figure 19). He has 23 pairs 

 of autosomes, an X, and a small Y.^ 



Subtype j. The male has an X, and a Y that Is of differ- 

 ent form and function from X (figure 7). 



Subtype 4. The Y chromosome does not differ in form or 

 size from X, but experiments of the kind to be described later 

 show that it differs from X In function. 



Other conditions, not classifiable under any of these four 

 subtypes, occur in some organisms. 



