22 GENETICS 



8. But the male has one pair in which one of the chromo- 

 somes is smaller or of a different form, or is lacking en- 

 tirely. This is the chromosome that we have called Y; it 

 comes from the father only. 



g. In place of this unequal pair the female has a cor- 

 responding equal pair of large chromosomes. The large 

 one that corresponds to the Y of the male we have called 

 X. But since the other one of this pair is just like it, we 

 must call it X, too. That is, the female has a pair of X-chro- 

 mosomes. 



10. The corresponding pair in the male has only one of 

 these large chromosomes that we call X, with or without 

 another of different form that we call Y. 



//. Thus the correct way to express the difference 

 between the sexes is that the female has two X-chromo- 

 somes, while the male has but one X-chromosome (accom- 

 panied in some species by a Y-chromosome). 



Substitution of One Chromosome for Another. — Keep- 

 ing the above facts in mind, we can discover the answer 

 to the second question proposed above. That question was: 

 What difference does it make if we substitute one chromo- 

 some for another? We discover that it may make a great 

 difference. We discover that by different substitutions of 

 chromosomes we can change the organisms in a great many 

 ways. We discover that different X-chromosomes have very 

 different effects on development. That is, different X-chro- 

 mosomes are physiologically diverse in their action, produc- 

 ing individuals with different characteristics. 



This discovery is made possible by the following facts, 

 which must be thoroughly fixed in mind — (see figures 4 

 and 5): 



1. The mother gives one of her two X-chromosomes to 

 each child. 



2. The father gives his X-chromosome to half his off- 

 spring, his Y-chromosome to the other half. 



