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24%

which allowed all four genotypes of progeny to be identified.

The percentage of recombinant progeny allowed the h- and r-mutants to be mapped.

a which allowed all four genotypes of progeny to be identified.

The percentage of recombinant progeny allowed the h- and r-mutants to be mapped.

total plaques total plaques

Conclusion: The recombination frequency indicates that the distance between h and r genes is 24%.

Phenotype

Progeny phage produced from

Genotype

Clear and small Cloudy and large Cloudy and small Clear and large h-r+ h+r-h+r+ h-r-

were then packaged into new phage particles. When the cells lysed, the recombinant phages were released, along with the nonrecombinant h+ rT phages and hT r+ phages.

Hershey and Rotman diluted and plated the progeny phages on a bacterial lawn that consisted of a mixture of B and B/2 cells. Phages carrying the h+ allele (which conferred the ability to infect only B cells) produced a cloudy plaque because the B/2 cells did not lyse. Phages carrying the hT allele produced a clear plaque because all the cells within the plaque were lysed. The r+ phages produced small plaques, whereas the rT phages produced large plaques. The genotypes of these progeny phages could therefore be determined by the appearance of the plaque (see Figure 8.25 and Table 8.4).

In this type of phage cross, the recombination frequency (RF) between the two genes can be calculated by using the following formula:

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