The evidence that crossing over occurs at the | vedclass

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(A) The evidence that crossing over occurs at the four-stranded stage (pachytene of meiosis-$I$) is provided by the analysis of ascospores in the fung

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$2 : 2 : 2 : 2$ arrangement of ascospores in Neurospora

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(A) The evidence that crossing over occurs at the four-stranded stage (pachytene of meiosis-$I$) is provided by the analysis of ascospores in the fungus $Neurospora$ $crassa$.In $Neurospora$,the linear arrangement of ascospores in the ascus reflects the segregation pattern of alleles.If crossing over occurred at the two-stranded stage,the resulting arrangement would be $4 : 4$.However,the observed $2 : 2 : 2 : 2$ arrangement of ascospores confirms that crossing over takes place after the chromosomes have replicated into four chromatids (four-stranded stage).

The evidence that crossing over occurs at the four-stranded stage and not at the two-stranded stage of the chromosomes comes from:

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