Question

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

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

Answer 1

Answer: (C)

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.