2. In Neurospora crasa tetrad analysis showed following result + : m :: 6 : 2. The phenomenon involved for above result would be
(1) Branch migration (2) Gene conversion
(3) Recombination (4) DNA replication
Question and Correct Answer
In Neurospora crassa tetrad analysis showing a + : m ratio of 6:2, the phenomenon involved is gene conversion. This aberrant segregation deviates from the expected 4:4 Mendelian ratio observed in ordered tetrads of Neurospora, where ascospores reflect meiotic products post-mitosis. Gene conversion occurs during homologous recombination, leading to non-reciprocal allele transfer and ratios like 6:2 or 2:6.
Option Analysis
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(1) Branch migration: This process involves base pair exchange at Holliday junctions during recombination, moving the branch point along DNA strands without altering allele ratios to 6:2. It contributes to heteroduplex formation but does not cause the non-Mendelian segregation seen here.
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(2) Gene conversion: Correct answer. Mismatch repair in heteroduplex DNA during meiosis converts one allele to another, producing 6 wild-type (+) to 2 mutant (m) ascospores instead of 4:4. In Neurospora, tetrad dissection reveals this as 3:1 at meiosis followed by mitosis.
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(3) Recombination: General crossing over between homologs yields 4:4 ratios in parental ditypes or tetratypes, not aberrant 6:2. It explains reciprocal exchanges but not non-reciprocal conversion.
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(4) DNA replication: Semi-conservative replication post-meiosis produces paired ascospores but maintains expected ratios without causing conversion. It does not alter segregation patterns.
