Linkage 105 



above a test is made as to the cross-over \'alue between 

 A and T. This value is discovered to be 20 per cent. 

 With these two cross-over values determined, consider- 

 able interest now becomes focused upon the relations of 

 A and R. Suitable tests reveal the fact that the cross- 

 over value between A and R is 30 per cent. When three 

 such cross-over values as these are considered together, 

 Morgan's theoretical scheme of the 'linear arrange- 

 ment of genes" is immediately suggested, for these 

 results can best be interpreted on the following basis. 

 The genes are arranged in a line on the chromosome, 

 and the frequency of crossing over between any two 

 genes depends upon their distance apart. Using arbi- 

 trary units to correspond with the actual percentage 

 of crossing over, the three genes employed in the example 

 may be arranged in the order ATR, with A and T 20 

 units apart, T and R 10 units apart, and A and R, there- 

 fore, 30 units apart. (The chromosome map first de- 

 vised, e.g., fig. 18, would then have to be modified some- 

 what to fit these new facts.) 



It is in this manner that Morgan and his students 

 have been able to construct rather startling chromo- 

 some maps, indicating in a very exact way the relative 

 position and spacing of scores of genes on a single chro- 

 mosome. Striking confirmation of the scheme appears 

 in the following fact. The fruit fly possesses four chro- 

 mosome pairs, one of which is visibly much shorter than 

 the others. Breeding results reveal that the many known 

 genes are associated in just four ''linkage groups," one 

 of the four groups containing a much smaller number 

 of genes than the other three, and being distinctly 

 ''shorter," as mapped from the cross-over values. In 



