Chapter 24 



INTRACHROMOSOMAL ABERRATIONS 



We have regarded each chromosome as a rather stable unit of 

 definite size and shape and we have assumed that the chromo- 

 some number of each plant and animal is 2n in the somatic cells 

 of the animal body and plant sporophyte and n in gametes, 

 spores, and gametophytes, but we have also mentioned upon 

 several occasions that there are exceptions to these principles. 

 We have pointed out that the somatic chromosome number is 

 not always 2n, and we have cited a number of examples that 

 show definitely that a chromosome is neither unchanging nor 

 indestructible. In the next few chapters we discuss such situ- 

 ations more thoroughly and indicate some of the effects they 

 have had on evolution. That the chromosomes are fixed in their 

 structure and that they always occur either in the diploid or 

 haploid condition, except for the endosperm of angiosperms, 

 where they are triploid, are among the earlier concepts of genet- 

 ics. Therefore, these other situations are regarded as chro- 

 Tnosomal aberrations. Chromosomal aberrations are of three 

 types, involving pieces of chromosomes, whole chromosomes, and 

 whole genomes. In this chapter, we consider the first type. 



Deficiency 



A deficiency is a chromosomal aberration in which a segment 

 of a chromosome is missing. A chromosome with a deficiency, 

 therefore, is not a complete chromosome so that a deficiency in 

 one or more chromosomes of a set results in a deficient genome. 

 We have discussed some important points in regard to deficien- 

 cies in Chapters 2, 4, 5, 12, and 16. It might be well to sum- 

 marize those points here and to add some information. 



If the missing portion is at the end of a chromosome, the 

 aberration is known as a terminal deficiency, but if it occurs at 

 any other place, it is an intercalary deficiency or a deletion. 

 Terminal deficiencies are considerably less common than dele- 



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