Linkage and Crossing Over Between Genes 



127 



Maize workers (/. to r.) at Cornell University in 1929: C. R. Burnham, 

 M. M. Rhoades, G. W. Beadle {crouched), R. A. Emerson, and Barbara 

 McClintock. 



SUMMARY AND CONCLUSIONS 



The nonallelic genes in a given chromosome are linked and tend to be transmitted 

 together to the next generation. Just as intergenic linkage produces an exception to 

 independent segregation, crossing over produces an exception to linkage between non- 

 alleles and causes linkage to be incomplete. Incomplete linkage proves that a chromo- 

 some contains more than one gene. In any given case, the degree to which linkage is 

 incomplete — as measured by crossover frequency — is constant and independent both 

 of the specific alleles which are present at the two different loci and of the gene com- 

 binations that enter the individual forming the gametes. Moreover, reciprocal cross- 

 over types are equally frequent. The crossover frequency in different cases is found to 

 vary considerably. 



A crossover chromosome is derived from a tetrad in which a crossing over between 

 the linked genes showing recombination involves only two of the four strands. For 

 closely linked genes, the crossover frequency is one half the frequency with which a 

 chiasma or a crossing over occurs between their loci. 



It is hypothesized that crossover frequency is directly related to the distance between 

 genes in a chromosome. One unit of crossover distance between genes is defined as 

 one crossover per one hundred postmeiotic cells (spores or gametes). Since different 

 genes linked to the same gene show different percentages of crossing over with this 

 gene, they are presumably different distances from it. 



