1 8 GENETICS 



chromosomes is lost or is lacking. Some of the observations 

 and experiments that prove this to be true will be taken up 

 in later sections. 



This discovery about sex and its relation to chromosomes 

 was the first great step in finding out certainly what is the 

 material on which heredity depends, and it made possible 

 all the others. To that extent the question is answered: 

 What difference does it make if one chromosome is re- 

 moved from the egg cell? The answer is that it changes a 

 female into a male. It brings about all the differences that 

 there are between the two sexes. This proves that the chro- 

 mosomes are indeed materials of heredity, in the sense that 

 changing them alters the characteristics of the Individual. 



This was a very great discovery. And it was still greater 

 because it opened the way to answering other questions. 

 In particular, it opened the way to answering the second 

 question: What difference will it make if one chromosome 

 be substituted for another? For it gave, as will be seen 

 later, a method of tracing a particular single chromosome 

 — one derived from the father — through a long series of 

 generations, knowing In which of the descendants it is pres- 

 ent, and in which it is replaced by some other chromosome. 

 And so It can be determined whether that particular chromo- 

 some has effects that others do not have. 



This is what led to the unraveling of the whole tissue 

 of heredity, in its relation to the materials passed from 

 parents to offspring. The facts and relations must there- 

 fore be examined carefully. But before this is done, cer- 

 tain further facts about the relation of sex to chromo- 

 somes must be considered. These are of great Interest In 

 themselves, and they must be known If one Is to under- 

 stand how the further facts of heredity were discovered. 



As just set forth, in some organisms the differences be- 

 tween males and females are due to the fact that the male 

 lacks one chromosome that Is present in the female. This 



