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CHAPTER 16 



with a chiasma are depicted in Figure 16-5 

 in somewhat more detail than those which 

 are shown in Figure 6-8B (p. 46). Stage I 

 shows a pair of homologous chromosomes, 

 one member (hollow bar) carrying the reces- 

 sives a and b, and the other (solid bar) 

 carrying their normal alleles. The black dots 

 represent centromeres. The homologs syn- 

 apse, form a tetrad (each univalent now repre- 

 sented by two sister strands), and give the 

 appearance at diplonema depicted in stage II. 

 Here there is a chiasma between the a and b 

 loci (the places in a chromosome containing 

 the genes). Note, when the univalents are 

 initially identical in appearance, that a chi- 

 asma results in the physical exchange of 



exactly equivalent segments between two non- 

 sister strands of a tetrad, the strands exchang- 

 ing segments being just as long after the ex- 

 change as they were before. Stage III shows 

 the dyads present after the first meiotic divi- 

 sion is completed. Note that the upper cell 

 contains one -\ — \- noncrossover strand and 

 one + b crossover strand, while the bottom 

 cell contains the reciprocal crossover strand 

 a + and the noncrossover strand a b. Stage 

 IV shows the four haploid cells produced 

 after the dyads form monads and the second 

 meiotic division is completed. Notice that 

 if one chiasma occurs in any position between 

 the loci of a and b, two haploid nuclei are 

 produced, containing noncrossover, parental 



FIGURE 16-5. The genetic consequences expected following a cliiasma between linked genes. 



Ill 



