98 CELL HEREDITY 



C B A 



n V Broken end 



C B A 



-- " ^Tn Fusion of broken ends after 



C B A 



-J chromosome duplication 



C B A A B C 



Anaphase 



C B A A B C 



X Telophase x i 



C B A A B C 



DJ Prophase (C 



C B A A B C 



Anaphase .fC^ * \" 



CBAAAABC C B B C 



{) X X n:i: Telophase rrr^ir 



C B A A A A B C C B B C 



FIGURE 4.3. The chromatid type of breakage-fusion-bridge cycle. An illustration 

 of the method by which variegation may be produced in tissues carrying a chromo- 

 some with a broken end. The dominant genes A, 6, and C are carried by the arm 

 with the broken end. The homologue of this chromosome (not diagrammed) is con- 

 sidered to be normal and to carry the genes a, b, and c. Duplication of this 

 broken chromosome results in fusion at the ends, giving rise to a dicentric 

 chromosome which breaks at the following anaphase. The arrow points to the 

 position of breakage; the two broken chromosomes enter sister telephase nuclei. 

 This process is repeated in successive mitoses, leading to deletion or duplication of 

 segments of genetic material in particular nuclei (after B. McClintock, 1941, 

 Genetics 26:234). 



McClintock provides a valuable foundation upon which further studies 

 can be constructed. 



Aberrations resulting from X-irradiation, ring chromosomes, and the 

 breakage-fusion-bridge cycle have provided some of the best material 

 for studying the effects of deletions and duplications of bits of chromo- 

 somal material upon phenotypic expression of particular genes and upon 

 viability. Many recessive alleles seem to exert no influence upon the 

 phenotype, but only a comparison between recessivity and deficiency 

 can provide critical evidence on this point. At the eye-color locus, 



