584 



COLLEGE ZOOLOGY 





Mal( 



Female 



Figure 411. Fruit flies, Drosophila melaiiogaster, and chromosomes. These httle flies have con- 

 tributed more to our knowledge of genetics than any other animal. Much enlarged. Diploid sets 

 of chromosomes (number found in somatic cells). Sex chromosomes are marked XY and XX. 

 This is known as the XY type of sex determination. 



Primordial germ cells 



Gametes of parents 



F, Ratio of sexes 1:1 



Male 



Female 



Figure 412. Diagram to show how sex is determined in man. The autosomes, which are the 

 same in both sexes, have been omitted for purposes of simplification. Although a 1:1 ratio of 

 sexes is expected, actually more males than females are born; the reason for this is still obscure. 

 The mechanism that explains sex in man is not universal as to details, yet the fundamentals 

 appear to be the same in organisms in which the sexes are separate. 



egg that is fertilized by the sperm with an 

 X chromosome becomes a zygote with two 

 X chromosomes and develops into a female. 

 Research on sex determination in the fruit 

 fly (Fig. 411) gives evidence that sex genes 

 are located in the X chromosomes and also 

 in the autosomes, and that in its final analy- 

 sis sex determination is a matter of genie 

 balance. It appears that the presence of two 



X chromosomes turns the balance in the 

 direction of the female sex, and the presence 

 of only one X chromosome results in the 

 development of the male sex. 



Sex-linked inheritance 



The X chromosome, in addition to bear- 

 ing genes influencing sex, also carries genes 



