Okay. So now let's talk about bacteriophages and mapping. Bacteriophages can actually be used to map bacteriophage genes or genomes. So, how do you do this? It's essentially the same way as with humans or other types, such as plants. You use recombination frequencies. Here is how you do this; you take a bacterial culture—they infect bacteria. You perform mixed infection, which means that you have two strains of bacteriophages that have different phenotypes, you mix them together, and infect this one bacterial culture. In this case, we have Virus 1, which has the H+ and R+ genes, and Virus 2, which has H− and R−. If it has H+, it's going to create purple colonies, and if it has R+, it's going to create small colonies. So now we can phenotypically look at the bacteria for the parental and the recombined. The parental will be purple small, and the other parental will be not purple large. The recombinants would be purple large or non-purple small.
We are looking for mixtures of positive and negative signs in the genotypes. The recombination frequency, instead of looking at plants, seeds, animals, or whatever the offspring itself, is observed based on the effect on the bacteria. Here we count the number of recombinant colonies versus the total number of colonies. As an example, let's consider these two viruses: H+R+ and H−R−. The h+r+ will show two parental phenotypes. If there are two recombinant phenotypes, such as purple large or black small, these can only form through recombination. You can use the recombination frequencies to map the bacterial genes and determine their proximity. The closer they are, the smaller the recombination frequency. In this case, we count six recombinants out of twelve trials, a 50% frequency, indicating that these genes are very far apart.
Bacteriophages have a unique form of recombination called intragenic recombination, which does not occur in humans or plants; entire genes can recombine, but not just parts of a gene. However, in bacteriophages, portions of a gene can recombine. This ability was studied by a scientist named Fenzer, particularly at the r11 locus of the T4 bacteriophage. Over 20,000 independent r11 mutants were collected, and he crossed them to analyze recombination. Through his study, he determined distances between mutations in terms of recombination frequency. This tedious process allowed him to identify specific mutations within a gene, marking a significant milestone in genetics. Today, this mapping can be done much more easily through sequencing, but at the time, it was a novel and crucial approach to understanding recombination frequencies and mapping bacteriophage genetics.
With that, let's move on.