58 GENETICS 



Group II — The females have a pair of dissimilar chro- 

 mosomes, in addition to the usual pairs of autosomes, while 

 the males have a corresponding pair of similar chromo- 

 somes, in addition to the autosomes. The dissimilar chro- 

 mosomes of the female are often called X and Y, as in the 

 other group, while the similar ones of the male are called 

 XX. But by some authors, the dissimilar pair in this group 

 are designated ZW, while the corresponding similar pair 

 are ZZ. 



In Group II, the females are heterogametic, producing 

 two types of ova (autosomes + Z) and (autosomes + W). 

 In the males, all the sperms are alike, with the constitution 

 (autosomes + Z). The sex Is determined by the type of 

 ovum, not by the type of sperm, that enters into the fer- 

 tilized egg. A sperm (autosomes + Z) uniting with an 

 ovum (autosomes + Z) gives a male (2 autosome sets -\- 

 ZZ). The same type of sperm (autosomes -j- Z) uniting 

 with an ovum (autosomes + W) yields a female (2 auto- 

 some sets + ZW). 



To this group belong birds and certain moths. The rela- 

 tions of the chromosomes to sex have been less fully studied 

 in this group than in Group I. 



2. Physiology of Sex Determination in Group I 



The conditions in Group I have been extensively studied; 

 they throw much light on the nature of the physiological 

 action of the chromosomes, in their effects on development. 



In Group I, as before seen, the males and females both 

 contain in their cells two sets of autosomes (one set derived 

 from each parent), and also an X-chromosome. They differ 

 in the fact that the female contains an additional X, while 

 the male has in place of this a Y; or in some species the 

 Y is lacking entirely. 



What part do these combinations of chromosomes play 



