308 



Cell as Basts oj Organic Activity 



The Chromosomal Mechanism. — The number of chromosomes 

 of a particular species of animals is constant. This number is formed 

 of the sex chromosomes and the autosomes. A normal animal has at 

 most two sex chromosomes while the number of autosomes varies from 

 species to species. The sex of an animal appears to be determined in 

 part by the balance between the sex chromosomes and the autosomes. 



In most animals, including mammals, the sex chromosomes are 

 designated as either X or Y. In general, the Y chromosome is dif- 

 ferentiated from the X by its smaller size. The female has two X chro- 

 mosomes, while the male has a single X and a single Y chromosome. 

 (Fig. 103). In some species the Y chromosome is lacking, but it is of 

 little importance since it ordinarily does not carry many genetic factors. 

 From this it can be seen that the sex of the individual is determined at 

 fertilization, as one-half of the sperms carry an X chromosome and the 

 other half carry a Y chromosome. The eggs will all have only X chromo- 

 somes. When the sperm and egg unite, a female will be produced if 

 the sperm entering carries an X chromosome ; a male will be produced 

 if this sperm carries a Y. It is apparent then that in normal populations 

 there should be approximately equal numbers of males and females. 

 This sex determination is illustrated in Fig. 97. 



Fig. 97. — Determination of sex. An individual with XX is a female, and one with 

 XY is a male; thus the type of sperm determines the sex of the offspring. 



At times Drosophila develop that have more autosomes or X chro- 

 mosomes than normal. These are ordinarily produced by the failure 

 of the chromosomes to pull apart during meiosis. If there are three 

 X chromosomes and a normal number of autosomes, a super female 

 is produced. If there is but one X chromosome and more autosomes 

 than normal, a super male will be produced. These flies are usually 

 larger, but they are less viable and frequently sterile. 



Intersexes may also be produced. These have more autosomes 

 than normal, but only two X chromosomes. Occasionally in early 

 embryology, the chromosomes will fail to divide properly and one side 

 of the animal will have a balance producing a female and the other 



