CHAPTER XIII 

 SEX DETERMINATION 



Sex determination is very properly a part of the sub- 

 ject of genetics. It should be realized, however, that a 

 vast amount of investigation has been carried on in this 

 field, and it will be possible here to take up only a limited 

 number of representative cases. 



Since this subject has been investigated a great deal 

 more thoroughly and for a great many more years in 

 animals than in plants, it will be appropriate first to 

 consider some of the findings of the zoologists. Until 

 very recently, at least, there appeared two outstanding 

 and seemingly quite contradictory views as to the basis 

 of sex determination. 



1. Some believed that sex is predetermined by the 

 chromosome equipment that enters into the zygote. 



2. Others believed that sex may be determined othcr- 

 w^ise than by the chromosomes, the decisive factors being 

 certain physiological conditions surrounding the unfer- 

 tilized egg or the developing embryo. 



These two general views will ])e referred to as the chro- 

 mosome theories and the physiological theories. Repre- 

 sentative examples of each will be considered brietly. 



Chromosome theories. — A classic example of the 

 simplest kind is to be found in the nematode worms. 

 Fig. 28 will indicate how sex is determined in this case. 

 Both male and female have ten chromosomes (com- 

 monly called aulosomcs) to determine most of their 



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