Question

Two theoretical genetic strains of a virus \(\left(a^{-} b^{-} c^{-} \text {and } a^{+} b^{+} c^{+}\right)\) were used to simultaneously infect a culture of host bacteria. Of 10,000 plaques scored, the following genotypes were observed. Determine the genetic map of these three genes on the viral chromosome. Decide whether interference was positive or negative.

Short answer

Answer: To determine the genetic map of the three genes and analyze the interference, we first calculate recombination frequencies for each pair of genes (a and b; a and c; b and c) using their observed genotypes. Then, we convert these values into map distances (in centimorgans, cM) and arrange the genes on a chromosome based on these distances. Finally, we calculate interference by comparing the observed double crossovers with the expected double crossovers. If the interference value is greater than 0, it is positive; if it's less than 0, it is negative.

Step-by-step solution

Interpret the given data

We are given 10,000 plaques scored with the observed genotypes for the viral strains. The question doesn't provide the genotypes and their observed occurrences, so let's assume that we have the following information: Genotype | Observed ---------|--------- \(a^{-}b^{-}c^{-}\) | A \(a^{+}b^{-}c^{-}\) | B \(a^{-}b^{+}c^{-}\) | C \(a^{-}b^{-}c^{+}\) | D \(a^{+}b^{+}c^{-}\) | E \(a^{+}b^{-}c^{+}\) | F \(a^{-}b^{+}c^{+}\) | G \(a^{+}b^{+}c^{+}\) | H Note that the summation of A to H should equal 10,000.

Calculate recombination frequencies

First, we need to calculate the recombination frequencies for each pair of genes (a and b; a and c; b and c) using the following formula: Recombination frequency = \(\frac{\text{Number of recombinant plaques}}{\text{Total number of plaques}}\) 1. Recombination frequency between gene a and b (RF_ab): RF_ab = \(\frac{B+C+E+F}{10,000}\) 2. Recombination frequency between gene a and c (RF_ac): RF_ac = \(\frac{D+F+G+H}{10,000}\) 3. Recombination frequency between gene b and c (RF_bc): RF_bc = \(\frac{C+D+G+H}{10,000}\)

Build the genetic map

Using the recombination frequencies (RF_ab, RF_ac, and RF_bc) calculated in Step 2, we can convert these values into map distance (in centimorgans, cM) using the following formula: Map distance (cM) = Recombination frequency × 100 1. Map distance between gene a and b (MD_ab): MD_ab = RF_ab × 100 2. Map distance between gene a and c (MD_ac): MD_ac = RF_ac × 100 3. Map distance between gene b and c (MD_bc): MD_bc = RF_bc × 100 We can now arrange the genes on a chromosome based on the map distances we calculated. The gene order is determined by the recombination frequencies, where the higher frequency corresponds to longer distances due to higher recombination.

Determine interference

The interference is a measure of deviation from the expected frequency of double crossovers. To determine if interference is positive or negative, we compare the observed double crossovers with the expected double crossovers. Expected double crossovers = (RF_ab × RF_bc) × Total plaques Observed double crossovers = Given in the table (either C, D, G, or H, depending on the gene order) Interference = 1 - (Observed double crossovers / Expected double crossovers) If the interference value is greater than 0, it is positive, which means that the occurrence of one crossover event reduces the likelihood of a second crossover event. If the interference value is less than 0, it is negative, meaning one crossover event increases the likelihood of a second crossover event occurring. Using the values provided in the table to calculate the interference, we can determine whether it's positive or negative.

Key concepts

Recombination Frequency

Recombination frequency is a critical measure in the field of genetics that quantifies the proportion of offspring with recombinant phenotypes resulting from a breeding experiment. In simpler terms, it’s a way to estimate how often a pair of genes are separated during the process of crossover in meiosis. This information is incredibly valuable for geneticists trying to determine the location of genes on a chromosome.

By observing the traits of offspring, such as the viral plaques in our textbook exercise, scientists can calculate the frequency at which different combinations of traits appear. Generally, the formula for recombination frequency is expressed as \( RF = \frac{Number \; of \; recombinants}{Total \; number \; of \; offspring} \).

A lower recombination frequency suggests that the genes in question are physically closer together on the chromosome, since they are less likely to be separated by recombination events during meiosis. Conversely, a higher recombination frequency implies that the genes are farther apart and more likely to be separated during genetic crossover.

Map Distance

Map distance is a concept that comes hand-in-hand with recombination frequency. It represents the relative positions of genes on a chromosome and is expressed in units known as centimorgans (cM). This measurement gives researchers a sense of the genetic ‘landscape’ — the further apart the genes are, the greater the map distance between them.

To figure out the map distance from recombination frequency, a simple multiplication is used: \( Map \; distance \; (cM) = Recombination \; frequency \times 100 \). If, for instance, the recombination frequency between two genes is 0.02 (or 2%), the map distance would be 2 centimorgans, indicating that these genes are relatively close to each other on the chromosome.

In the context of the exercise we’re analyzing, calculating map distances for different pairs of genes allows us to build a genetic map. This map is a crucial tool for understanding the layout of the viral genome and provides insights into how traits can be inherited.

Viral Genetics

Viral genetics is a fascinating and distinct field in genetics due to the unique properties of viruses. Unlike more complex organisms, viruses carry their genetic information in a compact and efficient manner, which can consist of either DNA or RNA. In studying viral genetics, scientists can unravel the intricacies of how viruses replicate, evolve, and interact with their host organisms.

In the given exercise, the focus is on the genetic composition of viral strains and how they infect host bacteria. By looking at the different genotypes that appear in the bacterial plaques, researchers can deduce the viral genetic map. The mapping techniques used for viruses are similar to those used for other organisms, but the smaller genomes and rapid reproductive cycles of viruses often make the analysis more straightforward and quicker.

Understanding the genetic structure of viruses is not only academically interesting; it is also critical for developing effective treatments and vaccines. By knowing which genes are responsible for virulence or other traits, scientists can target these areas when designing medical interventions.

Genetic Interference

Genetic interference refers to the phenomenon where the occurrence of one crossover during meiosis can affect the probability of a second crossover event happening nearby. It's an important concept because it influences the expected outcomes of genetic crosses and can complicate the creation of genetic maps.

Interference is calculated by comparing the observed double crossover frequency to the expected frequency, which is determined by the product of the individual recombination frequencies of the gene pairs involved: \( Expected \; double \; crossovers = (RF_{ab} \times RF_{bc}) \times Total \; number \; of \; offspring \).

If the observed number of double crossovers is less than what's expected, you have positive interference, indicating that one crossover event inhibits another from occurring nearby. Conversely, if more double crossovers are observed than expected, this suggests negative interference, where one crossover may actually facilitate another.

Understanding interference is vital for accurate genetic mapping. In the exercise, determining whether interference is positive or negative helps in confirming the correct gene order on the viral chromosome, providing a fuller picture of the genetic landscape.