298 



EVOLUTION AND GENETICS 



investigations have indicated that the chromosomes are not alone 

 responsible. 



In some species the male is characterized by an odd numlier of 

 chromosomes and the female by an even number. The odd 



MALE 



FEMALE 



Secondary 

 Oocyte 



Matured 

 Ovum 



Fig. 172. — Sex determination in species having heterogametic males. (From 



Walter.) 



chromosome in the male is without a synaptic mate and conse- 

 quently the reduction division produces two types of gametes. 

 Such species are said to have heterogametic males. The single 

 chromosome is called the x chromosome. Since the females have 

 an even number including two x chromosomes all female gametes 

 are the same. The sex of the individual is therefore determined 



