CHAPTER XIII 

 CHROMOSOMAL ABERRATIONS 



Exceptions to the normal mode of chromosome behavior are frequently 

 observed, particularly when large numbers of individuals are being 

 examined in experimental work. Often one is led to discover them by 

 suggestive abnormalities in the characters of one or more individuals. 

 Some types of aberration occur only rarely, while others appear much 

 more frequenth^; and it has been found that by various experimental 

 treatments, such as irradiation with X rays or ultraviolet light, the 

 frequency of occurrence can be markedly increased. Some aberrations 

 involve whole genomes and result in polyploidy (Chap. XIV). Others 

 affect a single whole chromosome, as in the nondisjunction leading to 

 the trisomic condition found so useful in assigning genes to their proper 

 chromosomes (page 174). Still others produce alterations involving 

 breakage of the chromosome. It is this last group of alterations that 

 will concern us in this chapter. 



AbnormaUties of several kinds have proved their value in cytogenetical 

 research, for not only do they furnish material for tests of hypotheses 

 founded upon normal chromosomal and genie behavior, but to certain 

 questions they often yield answers not obtainable in normal material. 

 They also throw a most interesting light upon the fundamental problem 

 of the nature of the gene. Finally, they afford important additional 

 clues to the role of intrachromosomal changes in the evolution of diversity 

 among organisms in nature. 



Deletion. — A deletion is the removal of a portion of a chromosome, 

 the remaining portion with the kinetochore then continuing as a deficient 

 chromosome (Fig. 139). A deleted segment, if it does not include the 

 kinetochore, cannot function in mitosis and is soon lost. Just how this 

 alteration is accomplished is not known, but the evidence indiiiates 

 that while two portions of a chromonema lie very close together two 

 breaks may occur there, the four freshly broken ends then reuniting 

 two by two in a new way. The loss of a portion usually leads to 

 the inviability of cells having no corresponding normal chromosome, 

 although there are some small deletions that are not lethal even in 

 monoploid cells. Deletions are nearly always intercalary, as shown in 

 the diagram; terminal deletions are relatively rare. 



193 



