396 INTRODUCTION TO EVOLUTION 



MUTATIONS 



Changes in the genetic materials are known as muta- 

 tions in the broad sense of that term. They may be conveniently divided 

 into those which produce visible changes in the chromosomes and those 

 which do not. Mutations involving visible changes in the chromosomes 

 are called chromosomal mutations or perhaps better chromosomal aber- 

 rations. Mutations which do not involve such visible changes and which 

 are presumably chemical changes in single genes are called gene mutations 

 (pp. 400-402). 



Chromosomal Aberrations 



Chromosomal aberrations are of two main types: (a) changes in 

 structure of individual chromosomes, and (b) changes in number of 

 chromosomes. 



Turning our attention to structural aberrations, we may note that a 

 piece of a chromosome may become detached and lost. Such a loss is 

 called a deletion or deficiency. The diagram in Fig. 17. 11 A represents a 

 normal chromosome. The dot near the center represents the centromere 

 to which the spindle fiber attaches. The letters of the alphabet represent 

 genes. The second diagram shows a deletion. The part of the chromosome 

 containing genes D, E, and F has become detached. Since it has no 

 spindle fiber attachment this fragment will probably be lost, though it 

 might possibly become attached to another chromosome. 



Deletions are harmful to their possessors. If an individual is heterozy- 

 gous for a small deletion (i.e., has one normal chromosome and one 

 deficient one, Fig. 17.11 ) that individual will probably be viable but is likely 

 to be abnormal in some way. For example, the missing genes D, E, and 

 F on one chromosome are likely to be compensated for, but usually not 

 completely so, by the corresponding genes in the other chromosome. On 

 the other hand, homozygosity for a deletion is likely to be lethal (e.g., 

 both chromosomes of a pair like the second one in Fig. 17.1 IB). Normal 

 viability requires that the full complement of genes be present. 



The opposite of a deletion is a duplication or repeat. In Fig. 17.1 IC the 

 section of chromosome containing the genes B and C is present twice. 

 The repeated section may have come from another chromosome that 

 suffered a deletion. Duplications do not necessarily lower the viability of 

 their possessors, although they may result in abnormalities of structure or 

 function. It is possible that repeats of this kind have been important in the 



