Question

Where does this evidence come from? – 'crossing over takes place at a four-stranded stage and not at two stranded stages of chromosomes'.

• A. 4 : 4 arrangement of ascospores in Neurospora
• B. Studies of meiosis in maize
• C. Studies on linkage maps of chromosomes in Drosophila
• D. 2 : 2 : 2 : 2 arrangement of ascospores in Neurospora

Answer 1

The correct option is D 2 : 2 : 2 : 2 arrangement of ascospores in Neurospora

During meiosis, crossing over happens at the four stranded stage. It happens in the pachytene stage of Prophase I. The process of interaction of genetic material, between the bivalent non-sister chromatids is named as crossing over. The evidence for this can be seen in the arrangement of ascospores in Neurospora.
Neurospora crassa is a type of red bread mold of the phylum Ascomycota. Neurospora produces 8 ascospores in a fruiting body, which are of 4 different types in the ratio of 2:2:2:2. Out of the 8 ascospores, 4 will be of parental type (2 of one parental type and 2 of another) whereas the other 4 will be recombinant types (2 recombinants from each parent).
The diagram shows the stepwise formation of 8 ascospores in an ascus of Neurospora.