CONSTITUTION OF THE CHROAIOSOMES 1 53 



way just described. Any two placed together give, not the 

 normal eye color, but an Intermediate condition. And all of 

 them have the same exchange ratio with any third gene. 

 Their exchange ratio with yellow body is 1.5 per cent, their 

 common exchange ratio with vermilion eye is 33 per cent. 

 They are all therefore different modifications or defects of 

 the same part of the chromosome, the same gene. This 

 series of eye colors has been mentioned before; it includes 

 white, ivory, buff, ecru, coral, eosin, tinged, cherry, blood. 



Such a series of diverse modifications of the same gene 

 is commonly spoken of as a set of multiple allelomorphs 

 or alleles. Many such sets of recessive multiple alleles are 

 known in different organisms. They all show the same pe- 

 culiarities. Any two (in different chromosomes) can be 

 brought together into the same individual, giving, not the 

 normal dominant characteristic, but a mixed or interme- 

 diate condition. And all the members of any such set have 

 the same exchange ratio with any third gene, indicating 

 that all are due to modifications of the same gene. 



All of this agrees, as will be seen, with the theory that 

 the genes are arranged in a consecutive series, and that dif- 

 ferent exchange ratios are the consequence of the different 

 distances between genes that are tested. Hence, character- 

 istics that are due to modifications of the same gene have 

 the same exchange ratios with any other; while character- 

 istics that are due to modifications of different genes have 

 diverse ratios with any other. 



6. There is a further important fact which again tests 

 the adequacy of this idea of the arrangement of the genes. 

 Two genes that are in different parts of the two chromo- 

 somes of the same pair can be brought by crossing-over 

 into the same single chromosome. Thus, in figure 35, by a 

 break and exchange of parts occurring In the region be- 

 tween the gene pairs aA and Kk, the two genes a and k 

 would be brought Into the same chromosome. On the other 



