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  Section: Genetics » Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
 
 
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Crossing over at four strand stage

 
     
 
Content
Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

In the last section, evidences for actual exchange of chromosome segments leading to crossing over were presented. One would, however, like to know whether exchange of segments takes place between entire chromosomes or between chromatids at four-strand stage. There is enough evidence which suggests that crossing over takes place at pachytene stage after the chromosomes have already duplicated giving rise to four strands, two strands belonging to each paired chromosome. The theoretical consequences of two strand and four strand crossing over are given in Figure 10.19. While two strand crossing over would lead to four recombinants, four strand crossing over will give two parental combinations and two recombinants among four gametes produced from a mother cell. By using attached X-chromosomes in Drosophila and also by studying linear order in which recombinants and parental combinations are found in an ascus of fungus Neurospora, it could be proved that crossing over takes place at four strand stage rather than at two strand stage.

Difference in consequences between crossing over at the two strand and four strand stage (redrawn from Strickberger's "Genetics")
Fig. 10.19. Difference in consequences between crossing over at the two strand and four strand stage (redrawn from Strickberger's "Genetics")
 
     
 
 
     




     
 
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