170 



( II \PTHR 12 



BREAKAGE 



ABCDEFG HIJ 



REPLICATION 



A B C D E F G HIJ 



AB CDE FGHIJ 



i u.i ri I 2 4 ( right ) . 

 Duplication ( tandem 

 type ) . 



CROSS-UNION 



ABCDECDE FG HIJ 



A B FGHIJ 



FIGURE 12-5 (below). Reciprocal transloca- 

 tion between nonhomologous chromosomes. 



K L M N O 



P Q R S T U 



ANEUCENTRIC TYPE: 

 K L M N O 



Q R S T U 



EUCENTRIC TYPE: 

 K L Q R S T U 



P M N O 



pericentric inversion always puts the hetero- 

 zygote at a reproductive disadvantage. For 

 this reason, only the smallest pericentric in- 

 versions — those which do not synapse when 

 heterozygous — are usually able to survive in 

 the wild. 



3. Duplication 



If, following two breaks in the same chro- 

 mosome, joining is delayed until after the 

 broken chromosome reproduces ( Figure 

 12-4). the two interstitial pieces and the 

 appropriate end pieces can join to produce 

 a eutelomcric chromosome with the inter- 

 stitial region repeated. This rearrangement 

 is called a duplication. Neither, either, or 

 both of the regions involved in the duplica- 

 tion can be inverted with respect to the orig- 

 inal arrangement. The two remaining end 

 pieces may or may not join to form a de- 

 ficient chromosome. (A deficiency can also 

 be produced without a duplication when the 

 end pieces join before chromosome replica- 

 tion.) Provided that the duplicated region 

 is small enough and acentric, it can survive 

 in nature. 



