You have a strain of Neurospora that is unable to synthesize histidine and thus

Question

You have a strain of Neurospora that is unable to synthesize histidine and thus requires H in the media in order to grow. You have isolated one revertant colony.Predict the expected proportion of the progeny that would be h+ if you cross the colony with the original mutant colony and the reversion occurred by each of the following mechanisms:

1) Precise change of the mutated base back to its original base.

2) A suppressor gene is mutated on a different chromosome

3) A suppressor gene is mutated on the same chromosome but 10mu distant from the mutated gene.

Explanation / Answer

1)

In true reversion, a mutant allele is converted back to wild type allele and is less common, while the forward mutation can be achieved by altering one of the bases. The chances of reversion are very low.   

In the given case, the reverted parent is crossed with mutant parent to obtain the offsprings. Generally, in Neurospora the sexual reproduction involves fusion of cell wall and nuclei and thus forms diploid nuclei. This undergoes meiosis followed by mitosis to form eight haploid cells. Each cell germinates and produces hyphae. Thus, 50% of the offsprings express the ability to synthesize histidine.

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