54 GENETICS 



individuals developed from the zygote. The zygote is 

 diploid: its chromosomes are In pairs, their total number 

 being 2«. The union of the gametes to form the zygote is 

 spoken of as the fertilization of the ovum. 



Heterogametic: The individuals of one sex (the male 

 in the types described above) form two kinds of gametes, 

 one containing an X-chroraosome, the other not. Such in- 

 dividuals are said to be heterogametic (or sometimes the 

 term digametic is employed in the same sense). 



Autosomes and Sex Chromosomes: The chromosomes X 

 and Y, since they play a prominent part in the production 

 of sex, are known as sex chromosomes. All the other 

 chromosomes are known as autosomes. Thus Drosophila 

 melanogaster (figure 7) has one pair of sex chromosomes 

 and three pairs of autosomes. 



NOTES AND REFERENCES ON CHAPTER II 



Extensive detailed accounts of the chromosomes, their history and 

 activities, will be found in text books of cytology, such as the following : 



E. B. Wilson (1928), The Cell in Development and Heredity. 

 Third Edition, 1232 pp.; L. W. Sharp (1934), Introduction to 

 Cytology. Third Edition, 567 pp. 



