CONSTITUTION OF THE CHROMOSOMES 1 37 



as to whether all these modifications are indeed in the same 

 part of the chromosome. 



We have before us then a definite suggestion for exam- 

 ination. This suggestion is that the chromosome is com- 

 posed of many diverse parts. Any of these parts may be- 

 come so modified as to produce a recessive character. But 

 if there is present in the cell that same part in the unmodi- 

 fied or dominant condition (in the other chromosome of the 

 pair), then the recessive character is not manifested. How 

 can we discover whether it is true that the chromosome 

 is composed of diverse parts? 



Evidence on this is obtained in another way. As we have 

 seen, the sex-linked characteristics resulting from modi- 

 fication of an X-chromosome follow from generation to 

 generation that X-chromosome. Observation shows an ex- 

 tremely important fact. Sometifnes two or more diverse sex- 

 linked characteristics follow the same X-chromosome. That 

 is, a single X-chromosome is so modified as to bring about 

 two diverse sex-linked characteristics, in the individuals 

 that carry it. 



For example, certain X-chromosomes cause in the fruit- 

 fly the eyes to be white, in place of the normal red. Certain 

 other X-chromosomes cause the body to be yellow, in place 

 of the normal gray. And sometimes we find individuals that 

 have both peculiarities: they have white eyes and yellow 

 bodies. Some of these individuals are males, having but one 

 X-chromosome, so that the two peculiarities must both be 

 due to the same X-chromosome. 



And if we breed such male individuals, we find that both 

 these peculiarities are indeed due to the same X-chromo- 

 some. Wherever the male's X-chromosome goes, these two 

 peculiarities go with it. They follow that single chromosome 

 for generation after generation, in the complicated course 

 that is followed by the X-chromosome. And in other cases, 



