The Cellular Basis 187 



the characters which are sex-linked are probably located in these 

 chromosomes ; the genes of the second and third groups of char- 

 acters are presumably in the other large chromosomes, while the 

 fourth group of only two characters probably have their genes 

 in the very small chromosomes (Fig. 67). If this interpretation 

 is correct, linkage is due to the grouping together of certain 

 genes in certain chromosomes, there are as many groups of 

 characters as there are pairs of chromosomes and as long as the 

 chromosomes preserve their individuality the linkage of genes in 

 the chromosomes and of characters in the developed organism 

 will persist. 



c. "Cross-Overs." But linkage of inherited characters is not 

 quite so simple as this statement would indicate for an extensive 

 study of this phenomenon in Drosophila has shown that while 

 characters are usually inherited in the same groups this is not al- 

 ways true. For example Morgan has found that when a female 

 fly with white eyes and yellow wings is crossed to a male with 

 red eyes and gray wings all the sons are yellow and have white 

 eyes while all the daughters are gray and have red eyes, but when 

 these latter are crossed about 99 per cent of the offspring show 

 the same linkage of the colors yellow-white, gray-red, but in i 

 per cent the linkage is yellow-red, gray-white. This interchange 

 of characters in the two groups, or "cross-over" as Morgan calls 

 it, may be explained by assuming that there has been an inter- 

 change of genes between the two sex chromosomes of the female. 

 When the paired chromosomes lie side by side in synapsis it is 

 known that they sometimes twist around each other and if in their 

 subsequent separation each chromosome should break at the point 

 where the two cross and a portion of one chromosome should be 

 joined to the other one we would have a relatively simple expla- 

 nation of the interchange or "cross-over" of genes and conse- 

 quently of the breaking up of the old group of characters and the 

 establishment of a new group (Fig. 66). 



Similar interchange of characters takes place in each of the 

 other three groups of Drosophila, and can be explained in the 

 same way. If a pair of chromosomes are twisted round each other 



