M. M. RHOADES 



University of Illinois 



Chapter 4 



Preferential 

 Segregation in Maize 



The outstanding example of the utilization of heterosis in plant improve- 

 ment is that of hybrid corn. Extensive studies on maize genetics have clearly 

 demonstrated that chromosome and gene segregation are in accordance with 

 Mendel's laws of segregation and recombination. It would appear, therefore, 

 that any unusual mechanism operating in maize to produce deviations from 

 normal Mendelian behavior should be worthy of our consideration, even 

 though the principles involved have no bearing on the nature or manifesta- 

 tion of heterosis. The purpose of this section is to present data on preferential 

 segregation in maize and to offer a tentative interpretation of this phe- 

 nomenon. 



Two kinds of chromosome 10, the shortest member of the haploid set of 

 ten, are found in populations of maize. The common or normal type gives 

 typical Mendelian ratios when the two homologues are heterozygous for 

 mutant loci. The second kind of chromosome 10, which has been found in a 

 number of races from Latin America and the southwestern United States, 

 also gives normal Mendelian ratios for chromosome 10 loci in plants homozy- 

 gous for this chromosome. This second or abnormal kind of chromosome 10 

 differs from the normal chromosome 10 by a large, chiefly heterochromatic 

 segment of chromatin attached to the end of the long arm and also in the 

 chromomeric structure of the distal one-sixth of the long arm (see Fig. 4.1 

 and Fig. 1 of Plate I). As is illustrated in Figure 4.1 the chromomeres in this 

 region are larger and more deeply staining than are the correspondingly 

 situated chromomeres of the normal homologue. 



Although normal Mendelian ratios are obtained for segregating loci in 

 chromosome 10 in both kinds of homozygotes, we were able to show in an 

 earlier paper (Rhoades, 1941) that preferential segregation occurs at mega- 



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