The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype. Experiment A Experiment B Experiment C phe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71% phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21% phe⁻ ala⁺ 19% leu⁻ ala⁺   0%   leu⁻ phe⁺ 0% phe⁺ ala⁺   3% leu⁺ ala⁺   4% leu⁺ phe⁺   3% In Experiment B, why are there no transductants with the genotype leu⁻ ala⁺?

Key Concepts

Generalized Transduction

Generalized transduction is a process by which a bacteriophage (phage) can transfer any segment of bacterial DNA from one bacterium to another. During this process, the phage mistakenly incorporates bacterial DNA into its own genome and, upon infecting a new host, can introduce this DNA into the recipient bacterium. This mechanism is crucial for understanding how genetic material is exchanged between bacterial strains, particularly in the context of the E. coli strains mentioned in the question.

Selection of Transductants

In genetic experiments, transductants are selected based on specific traits or markers, which are the phenotypic expressions of the underlying genotypes. In the context of the question, the selection for leu⁺, ala⁺, and phe⁺ indicates that only those bacteria that have acquired the corresponding genes from the donor strain are being counted. This selective pressure influences the observed frequencies of the unselected markers, as only certain combinations of genes will survive and proliferate under the given conditions.

Linkage and Genetic Mapping

Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. In the context of the experiments, the absence of leu⁻ ala⁺ transductants in Experiment B suggests that the genes for leucine and alanine biosynthesis may be closely linked, making it unlikely for both to be transferred together when selecting for ala⁺. Understanding genetic linkage is essential for interpreting the results of transduction experiments and predicting the outcomes based on the proximity of genes on the bacterial chromosome.