178 



Plant Genetics 



Chromosome theories. — A classic example of the 

 simplest kind is found in the nematode worms. Fig. 39 

 will indicate how sex is determined in this case. The 

 mechanism is obvious. Both male and female have ten 



MALE 



0000 



o • o 



00 QO 



MATURE INDIVIDUALS 



FEMALE 



0000 

 o^^ o 



OOP o 



00 00 

 o • o 

 00 00 



Zygotes 



00 00 

 o# •O 

 0000 



(Thb type of zygote 

 produces a male) 



(This lype of zygote 

 produces a female) 



Fig. 39. — Illustrating behavior of sex chromosomes 



chromosomes to determine most of their somatic char- 

 acters; but in addition there are extra chromosomes that 

 determine sex, known variously as sex chromosomes, 

 X chromosomes, and heterochromosomes. In this case the 

 male contains only one sex chromosome while the female 

 contains two. At the reduction division, coimected 

 with gamete formation, the chromosome equipment 



