CHAPTER IX 

 THE OEDER OF THE GENES 



The proof of the linear order of the genes is derived 

 di]-ectly from the linkage data. It is not dependent on the 

 chromosome theory of heredity. Fortunately, as was 

 ]^ointed out in the last chai)ter, there are many facts about 

 the maturation stages of the eggs and sperm that fit in 

 extraordinarily well with the theory of the linear order 

 of the genes, but let me repeat, the proof of the order is 

 not dependent on the chromosomal situation. The evi- 

 dence for the linear order is furnished by linkage and its 

 correlative phenomenon, crossing over. By linkage is 

 meant that certain factors that enter the cross from each 

 parent remain together in subsequent generations, more 

 often than they separate. For example, if in Drosophila 

 yellow wings and white eyes have entered from one parent 

 and gray wings and red eyes from the other, the new 

 (crossover) combinations, yellow and red, gray and 

 white, are less numerous than are the original linked 

 combinations, yellow and white, gray and red. The num- 

 ber of individuals (crossovers) that result from this 

 interchange, expressed as percentage of the whole number 

 of indi\dduals, is called tlie crossover value. Such a 

 percentage indicates how often the linkage is broken. 

 Thus, if crossing over between yellow and white is shown 

 in 1 per cent, of the gametes, then 1 stands for the cross- 

 over value of yellow and white. Conversely, yellow and 

 white have remained together (linked) in 99 per cent, of 

 the gametes. We speak of the linkage relations in such 

 cases in terms of the crossover values, here 1 per cent. 



For the proof of the linear order of the genes, it is only 



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