The Cellular Basis 193 



grouping together of certain genes in certain chromosomes, there 

 are as many groups of characters as there are pairs of chro- 

 mosomes and as long as the chromosomes preserve their identity 

 the linkage of genes in the chromosomes and of characters in the 

 developed organisms 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 linked in four constant 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, the genes for these characters being 

 linked together in the X chromosomes, all the sons are yellow 

 and have white eyes while all the daughters are gray and have red 

 eyes, gray wings and red eyes being dominant over yellow and 

 white; but when these F^s 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 interchange 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 explanation of the interchange or "cross-over" 

 of genes and consequently 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 



* Such "cross-overs" occur only in the female of Drosophila, though they 

 may occur in the males of other species. They occur when the synaptic 

 pairs of chromosomes are long, slender threads. 



