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Question

In Neurospora, arrangement of ascospores in ascus is 2a+ : 4A : 2a+ showing

A
No crossing over
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B
First division segregation
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C
Second division segregation or presence of crossing over
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D
Some meiosis
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Solution

The correct option is B Second division segregation or presence of crossing over
Neurospora is eight-spored heterothallic species. It has linearly arranged ascospore pairs. The order of ascospores shows the genetic events happened during meiosis. It also shows that crossing over occurs at the four-strand stage during meiosis. When a gene marker is close to the centromere, the two alleles are likely to segregate from one another at the first division of meiosis and result in a 4a+: 4A ascospore pattern for the segregating alleles. However, when a gene marker is far from the centromere, crossing over takes place and the two alleles segregate at the second division of meiosis. It results in 2a+: 2A : 2a+: 2A or 2a+: 4A : 2a+ ascospore pattern for the segregating alleles.
Thus, the correct answer is option C.

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