Consider the following genetic map of phage (a.ka. Moo Moo) where A, L, N and Z

Question

Consider the following genetic map of phage (a.ka. Moo Moo) where A, L, N and Z are essential genes and M affects plaque morphology (M+ = large plaque:M-= small plaque) M N When M+ and M-phage are crossed, ~50% of the progeny plaques are large and ~50% are small. However, a very small fraction of the progeny plaques have a "mottled" appearance; that is, the single plaque seems to consist of a mix of M+ and M phage. Examination of the phage isolated from individual mottled plaques indicates that indeed half are M+ and half are M. The following crosses are performed and the progeny plated at 30°C (permissive temperature for growth of ts mutants) and 42°C (non- permissive for ts mutants) Fraction of half-and-half plaques genotypes 30°C 42°C A's M. cross #1 10-4 10-3 cross #2 104 10-2 Where are the half-and-half plaques coming from, what explains the increased fractions at 42°C and why does this number increase even more for Cross 2?

Explanation / Answer

Based on the given data,

These half-and-half plaques are coming due to improper integration or recombination of phage into host genome in a single population. If we see the given genetic map, the map distance between the L and M genes and M and N is very closer than A and M. So, there is a chance of less recombination between the L and M and M and N genes when compared to A and M. Hence, the fraction of half-and-half plaques increases for cross 2.

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