398 INTRODUCTION TO EVOLUTION 



in one location on a chromosome does not necessarily have the same 

 action it would have in another position. (Gene D, Fig. 17.1 ID, when 

 located next to gene K may not have the same action it would have 

 when located next to gene C.) 



Inversions have another interesting genetic effect: they tend to suppress 

 crossing over (p. 395). In a heterozygote for an inversion (an individual 

 having in a certain pair one "normal" and one inverted chromosome) 

 normal pairing at synapsis is difficult and hence the likelihood of crossing 

 over is reduced. Thus inversions tend to cause chromosomes to remain 

 intact. This might have evolutionary significance since if a chromosome 

 came to contain a superior arrangement of genes it might be of advantage 

 not to have the arrangement destroyed through crossing over. At times 

 experimenters deliberately introduce inversions into their experimental 

 stocks so that the chromosome in which they are interested may remain 

 intact (pp. 460-464). 



Crossing over involves exchange of parts of homologous chromosomes, 

 chromosomes which constitute a pair. Sometimes part of a chromosome 

 may become detached, and then become attached to another chromosome 

 which is not homologous to the first. This is known as translocation. If 

 nonhomologous chromosomes exchange parts the exchange is known as 

 reciprocal translocation. A case is illustrated in Fig. 17.12; two chromo- 

 somes (not homologous, as evidenced by the differing genes) are shown 

 as exchanging their entire right "arms." Reciprocal translocation of this 

 type seems to have been important in the formation of varieties within 

 some species of plants. 



The second class of chromosomal aberrations involves changes in num- 

 ber of chromosomes. Rarely a whole chromosome may be lost and the 

 organism still survive. But loss of chromosomes is usually lethal, as we 

 have noted that loss of pieces of chromosomes (deletion) is likely to be. 

 Increase in number of chromosomes may occur at times and may have 

 genetic and evolutionary significance. 



A gamete may come to possess an extra chromosome by an error in 

 meiosis. In Fig. 17.3, p. 384, normal meiosis is shown. Suppose, however, 

 that when the secondary oocyte divided to form the ovum and second 

 polar body, the short pair of chromatids failed to separate and that both 

 were retained in the ovum. Such failure of chromatids to separate and be 

 distributed normally is called nondisjunction. As a result the ovum 

 would contain one long chromosome and two short ones. When fertilized 

 by a normal sperm the fertilized ovum would contain two long chromo- 

 somes but three short ones. Such an individual is called a trisomic (Fig. 



