Questions and Problems 411 



Chromosomal translocations have been worked out by Blakes- 

 lee and by Bergner for six species, including 681 races of D. 

 stramonium. 



B-Type Chromosomes 



In discussing fragments, we have pointed out that segments 

 of a chromosome that have a centromere behave as a normal 

 chromosome in both mitosis and meiosis except for difficulties of 

 chiasma formation resulting from their small size. A number of 

 such very small chromosomes are found in some strains of maize. 

 These chromosomes, which consist of a centromere and hetero- 

 chromatin with a much smaller amount of euchromatin, are 

 called B-type chromosomes. No genes have been identified in 

 them. Many strains do not have these small chromosomes, but 

 when they are present, they can be accumulated by breeding. 

 Randolph has by selection produced plants with as many as 

 thirty-four B-type chromosomes in addition to the twenty nor- 

 mal chromosomes. These chromosomes are not essential to 

 normal growth and reproduction and do not appear to have any 

 beneficial effects. Large numbers of B-type chromosomes, how- 

 ever, appear to bring about reduced fertility, decrease in vigor, 

 increase in the size of the pollen and of other reproductive struc- 

 tures, defective seeds, scarred endosperm, and an increase in cell 

 size. Although the origin of these chromosomes has not been 

 satisfactorily explained, it has been suggested that the effects of 

 these large numbers may be the result of genes located in the 

 euchromatic regions. By breakage of the B-type chromosomes, 

 smaller ones have been produced and are generally classified 

 according to their size. They range from the C-type, which is 

 shorter than the B's but still is elongated, to the F-type, which 

 is only slightly above the lower limit of visibility of the photo- 

 microscope. 



QUESTIONS AND PROBLEMS 



1. Two of the chromosomes in a certain plant are abcdefghij and 

 abghij. Show how they would pair at meiosis. If such a plant were 

 selfed, what would be the viable offspring? Would such heterozygotes 

 be more likely to be found among the offspring in plants or in animals? 

 Explain. 



