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.

Determine the order of genes on the donor chromosome.

Verified step by step guidance

1. Step 1: Understand the problem. The goal is to determine the order of genes (leu, phe, ala) on the donor chromosome based on the frequencies of co-transduction observed in the experiments. Co-transduction frequency is inversely proportional to the physical distance between genes on the chromosome.
2. Step 2: Analyze Experiment A (selection for leu⁺). The frequencies of phe⁺ and ala⁺ in the transductants indicate how closely these genes are linked to leu. Higher frequencies suggest closer linkage. For example, phe⁺ appears in 50% of transductants, while ala⁺ appears in only 19%, suggesting phe is closer to leu than ala.
3. Step 3: Analyze Experiment B (selection for ala⁺). The frequencies of leu⁺ and phe⁺ in the transductants indicate how closely these genes are linked to ala. Leu⁺ appears in 48% of transductants, while phe⁺ appears in 0%, suggesting leu is closer to ala than phe.
4. Step 4: Analyze Experiment C (selection for phe⁺). The frequencies of leu⁺ and ala⁺ in the transductants indicate how closely these genes are linked to phe. Leu⁺ appears in 21% of transductants, while ala⁺ appears in 0%, suggesting leu is closer to phe than ala.
5. Step 5: Combine the results from all experiments to deduce the gene order. Based on the co-transduction frequencies, the gene order on the donor chromosome is likely leu - phe - ala, with leu being closest to phe and ala being the farthest.

Key Concepts

Transduction

Transduction is a process of genetic transfer in bacteria where bacteriophages (viruses that infect bacteria) mediate the transfer of DNA from one bacterium to another. In generalized transduction, any bacterial gene can be transferred, allowing for the mapping of genes based on the frequency of co-transduction. Understanding this process is crucial for analyzing the results of the experiments involving the E. coli strains.

Gene Mapping

Gene mapping is the process of determining the relative positions of genes on a chromosome. In this context, the frequency of co-transduction events can be used to infer the order of genes based on how often they are transferred together. The closer two genes are on a chromosome, the higher the likelihood they will be co-transduced, which is essential for deducing their arrangement.

Genotype Frequencies

Genotype frequencies refer to the proportion of different genotypes in a population or sample. In the experiments described, the frequencies of various genotypes among the transductants provide insights into the genetic linkage and order of the genes. Analyzing these frequencies helps in determining which genes are more likely to be inherited together, thus revealing their relative positions on the chromosome.