CHROMOSOMES AND MEN DELI AN HEREDITY 



299 



that the frequency with which crossing-over occurs is a function of the 

 actual distance apart of the factor pairs in question (Sturtevant, 1913). 

 In other words, the farther apart two pairs of genes he, the greater the 

 chance for a crossover to occur between them. It was assumed that 

 when flies showed 1 per cent of recombinations after a back-cross, the two 

 pairs of genes concerned were one "unit" apart in the chromosome, and 

 so on for other percentages. Certain refinements have since been made in 

 this hypothesis, but linkage studies in many organisms, together with 



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Fig. 171. — Linkage map for Zea Mays, showing the arrangement of the genes as deter- 

 mined genetically. The length of the lines does not indicate actual chromosome length; 

 cf. Fig. 170. The position of genes marked with asterisks is only approximate. The 

 genes below the lines are known to belong to the linkage groups indicated, but they have 

 not yet been assigned to definite loci. {After R. A. Emerson, 1933.) 



evidence from translocations of portions of chromosomes (see next 

 chapter), have given it striking confirmation. 



The method of determining the relative position of genes by studying 

 their linkage relations may now be illustrated with a case in Zea (Fig. 172). 

 In chromosome pair II there are, among others, the following pairs of 

 genes = Lg Ig, for liguled vs. liguleless leaves; B b, for more intense vs. 

 less intense plant color; Sk sk, for normal vs. silkless ears. Between 

 Lg lg and B h (region a) the percentage of recombination was found to be 

 28.6, so that these pairs of genes were assigned to loci 28.6 units apart in 

 the "map" of the chromosomes. Between B b and Sk sk (region |8) the 



