T02 



CELL HEREDITY 



(c) 



FIGURE 4.5. Inversion heterozygote paired at pachytene. 



(o). The normal chromosome and its homologue with an inverted segment are 

 shown before duplication and meiotic pairing. 



(b). The configuration resulting from pairing of the homologous chromosomes of 

 (a) consists of on inversion loop. The progeny are viable so long as crossing 

 over does not occur within the loop. When it does occur, for example, between 

 regions 2 and 3, the results shown in (c) are obtained. 



(c). Consequences of crossing over within an inversion loop. If a crossover occurs 

 in the region between 2 and 3, the result will be the four chromosomes shown here. 

 Two are parental, and will give rise to viable offspring. The other two will give 

 rise to lethal progeny, since one chromosome is dicentric and will break in sub- 

 sequent divisions, and the other product is an acentric chromosome which will be lost. 



