Question

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

• A. 2: 2 : 2 : 2 arrangement of ascospores in Neurospora
• B. Studies of meiosis in maize
  
• C. Studies on linkage maps of chromosomes in Drosophila
  
• D. None of the above

Answer 1

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

Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" refers to the characetristic striations on the spores. An analysis of 939 asci in which the genotypes of all meitoic products (ascospores) could be determined found 11 asci with an exceptional segregation pattern. These included 6 asci in which there was one wild type meiotic product but no expected reciprocal double - mutant (B5B3) product. Furthermore, in three asci the ratio of meiotic products was 1B5:33B3, rather than in the expected 2:2 ratio.