studies of the chromosomes and by studies of Mendelian 

 inheritance. 



The X or Sex Chromosome. — In the study of sexual cells 

 of certain insects by Henking, McClung, Stevens, Wilson 

 and ethers it was observed that all the eggs and half of the 

 sperms contained an X or accessory chromosome. It was 

 observed also that when an egg was fertilized by a sperm 

 containing the X chromosome, a female was produced, but 

 when an egg was fertilized by a sperm without the X chro- 

 mosome, a male was produced. The female contained, 

 therefore, two X chromosomes and the male only one. 

 However, other observers have noted in some of the sperm - 

 cells of certain insects, many mammals and certain plants 

 the presence of the X chromosome, and in others a differ- 

 ently shaped chromosome which has been called the Y 

 chromosome. The Y chromosome, however, does not carry 

 any of the factors located in the X chromosome, and "does 

 not appear to have any effect upon the development of the 

 body characters, so that the male depends upon a single X 

 chromosome for the development of those characters deter- 

 mined by the factors borne by this chromosome. The Y 

 chromosome may, therefore be regarded as a neutral mate 

 for the X chromosome of the male" (Babcock and Clau- 

 sen). Morgan has called this the AT type of inheritance^ 

 (See also pages lll-112j. 



On the other hand, Doncaster and Raynor and others 

 have observed that in Abraxas (English Currant Moth), 

 chickens, ducks and canaries, all the sperms and half the e^gs 

 contain an accessory chromosome (called W to distinguish 

 it from the accessory X chromosome of the X Y type ) and 

 that the fertilized egg or zygote containing the 2 W chromo- 

 somes is a male, and the zygote with the 1 W chromosome is 

 a female. In some cases again, there appear to be evid- 

 ences of a chromosome (called the Z chromosome) in some 

 females, which may also be regarded as a neutral mate for 

 the W chromosome of the female. Morgan calls this the 

 WZ type of inheritance. (See also pages 11 5- 1 1(3) . 



The two types of sex inheritance, discussed above and 

 in chapter 15, are therefore, (1) one where the female is 

 homogametic with two X chromosomes, or, in Mendelian 

 terms homozygous for the sex factor, and the male hetero- 

 gametic and heterozygous; and (2) one where the male is 

 homogametic and homozygous, and the female hetero- 

 gametic and heterozygous. 



(])— This type of inheritance occurs in all mammals and in a number 

 of insects. 



142 



