Structural Changes Within Chromosomes 



153 



PARACENTRIC BREAKS 

 ABCDEFGHIJ 



1 r— 



A G H I J Deficiency 



B C D E F or B C^ £ 



F 



(a) 



lost 



AFEDCBG H 



Inversion 

 (b) 



PERICENTRIC BREAKS 

 ABCDEFGHIJ 



A B C D I J [ost IaBCDHGFEIJ 



-^— — — I I 



n I 



\^J Deficiency | 



G F 



Inversion 



(0 (d) 



FIGURE 19-2. Some consequences of two breaks in the same chromosome. 



production of deficiencies, that we have 

 ignored the usual consequence of two breaks, 

 which is that the ends from both breaks resti- 

 tute. Again, only the consequences of the 

 failure of breaks to restitute will be described 

 in the discussion that follows regarding the 

 production of other types of structural change. 

 Another structural consequence of two 

 breaks in the same chromosome is repre- 

 sented in Figure 19-2b and d. In this case, 

 the middle piece is inverted relative to the end 

 pieces and undergoes exchange unions with 

 them. The result is either a paracentric or a 

 pericentric inversion (Figure 19-2b and d, re- 

 spectively), which can occur by having the 



middle portion move while the ends are rela- 

 tively stationary, or the reverse. Note that 

 an inversion is a euploid rearrangement, and 

 is of no phenotypic consequence from this 

 standpoint. 



How would you expect an inversion to be- 

 have during meiosis? If an inversion occurs 

 by mutation and is retained in the germ line 

 of a population, it may become homozygous 

 in some individuals. In these individuals, 

 meiotic behavior will be normal whether or 

 not the tetrad is involved in chiasma forma- 

 tion, since all the strands involved are identi- 

 cally inverted. Other individuals, however, 

 may possess one inverted homolog and a 



