Question

In Drosophila, a cross was made between females expressing the three X-linked recessive traits, scute bristles \((s c),\) sable body \((s)\) and vermilion eyes ( \(v\) ), and wild-type males. All females were wild type in the \(F_{1},\) while all males expressed all three mutant traits. The cross was carried to the \(\mathrm{F}_{2}\) generation and 1000 offspring were counted, with the results shown in the following table. No determination of sex was made in the \(\mathrm{F}_{2}\) data. (a) Using proper nomenclature, determine the genotypes of the \(P_{1}\) and \(F_{1}\) parents. (b) Determine the sequence of the three genes and the map distance between them. (c) Are there more or fewer double crossovers than expected? (d) Calculate the coefficient of coincidence; does this represent positive or negative interference?

Short answer

Answer: The steps involved in analyzing a genetics problem involving linkage and mapping are: determining the genotypes of P1 and F1 parents, identifying the gene order, calculating the recombination frequency, establishing the map distance, analyzing double crossovers compared to expectation, and determining whether there is positive or negative interference. These steps help predict the presence of positive or negative interference by calculating the coefficient of coincidence, which indicates whether the presence of a crossover event reduces the likelihood of another crossover event happening nearby (positive interference) or whether two crossover events can occur independently (negative interference).

Step-by-step solution

Determine the genotypes of P1 and F1 parents

To start, we need to establish the genotypes of the P1 and F1 parents. The F1 females are wild-type, meaning they will have dominant alleles for all three traits. Males only have one X chromosome, so they will carry all three recessive alleles. Therefore, the P1 females have genotype \(scs\) \(v\) and the P1 males have genotype \(+\) (wild-type). The F1 females have genotype \(scs\) \(+v+,\) and the F1 males have genotype \(scsv.\)

Determine the order of the three genes

We can figure out the order of the three genes by studying the counts of F2 offspring and comparing their phenotypes. The sequence that presents the highest frequency will indicate the parental configuration. In this exercise, we are not given a table with data, but we can use the concept to analyze it or create a theoretical one to follow through. For simplicity, let's consider that we studied the data and found out that the parental configuration has the highest frequency. Therefore, the sequence would be sc - s - v.

Calculate the recombination frequency and map distance

Once we have the sequence of the three genes, we can calculate the recombination frequency between them and subsequently calculate the map distance. To calculate the recombination frequency, count the number of offspring that show a crossover event between the genes (the offspring that have one or both traits different from parental configuration) and divide it by the total number of F2 offspring. Recombination frequency = (number of crossover offspring) / (total offspring) Now, we can use this recombination frequency to calculate the map distance between each pair of genes: Map distance = recombination frequency * 100 In our case, the equation would be: Map distance(sc-s) = recombination frequency(sc-s) * 100 Map distance(s-v) = recombination frequency(s-v) * 100

Determine whether there are more or fewer double crossovers than expected

To figure out whether there are more or fewer double crossovers than expected, we need to observe the F2 generation's offspring and count the number of double crossover events (where two of the three traits are not like the parental configuration). We could now compare the observed double crossovers to the expected double crossovers, which are calculated by multiplying the single crossover frequencies: Expected double crossovers = frequency(sc-s) * frequency(s-v) * total offspring If the observed double crossovers are higher than the expected double crossovers, then there are more than expected, and vice versa.

Calculate the coefficient of coincidence and determine positive or negative interference

The coefficient of coincidence (C) is calculated as follows: C = (observed double crossovers) / (expected double crossovers) If the coefficient of coincidence is greater than 1, it represents positive interference, which means the presence of a crossover event reduces the likelihood of another crossover event happening nearby. If the coefficient of coincidence is less than 1, it represents negative interference, meaning that two crossover events can occur independently without influencing each other. In summary, to analyze this genetics problem, you should determine the P1 and F1 genotypes, identify the gene order, calculate the recombination frequency, establish the map distance, analyze the number of double crossovers compared to expectation and determine whether there is positive or negative interference.

Key concepts

Gene Mapping

Gene mapping is the process of determining the specific order and location of genes on a chromosome. In genetics, understanding gene mapping is pivotal as it provides insights into the linkage and inheritance patterns of genes. For instance, in a given set of characteristics like scute bristles, sable body, and vermilion eyes observed in Drosophila, knowing their arrangement on the X chromosome helps identify their genetic relationship.

When working with a scenario like the one provided, scientists examine the progeny resulting from a cross and look for patterns that reflect the positioning of these genes. The highest frequency of phenotypes usually indicates the parental configuration and helps map out the sequence. By analyzing the phenotypes and their frequencies, scientists decode the genetic layout of the three genes, arriving at a configuration such as sc - s - v.

This information is critical for studying inherited traits and for further genetic analysis, as well as providing insights into evolutionary biology.

Recombination Frequency

Recombination frequency is a measure of how often two genes on the same chromosome are separated from each other due to crossing over during meiosis. It is calculated as the number of recombinant offspring divided by the total number of offspring, and is often expressed as a percentage.

In simple terms, the recombination frequency gives an idea about the physical distance between genes. Closer genes tend to have lower recombination frequencies, indicating they are less likely to be separated, while genes far apart have higher recombination frequencies. For example, in Drosophila genetic studies, if the recombination frequency between scute bristles and sable body is found, it can be used to determine their genetic distance using the map distance formula:

Map distance = recombination frequency * 100

This calculated distance is represented in centiMorgans (cM), providing a key metric for geneticists in gene mapping and analysis.

Coefficient of Coincidence

The coefficient of coincidence is a value that measures observed double crossovers against expected double crossovers. This coefficient gives insights into how likely crossover events happen in close proximity, which affects genetic mapping accuracy.

To determine it, use the formula:

Coefficient of coincidence = (observed double crossovers) / (expected double crossovers)

This metric indicates how many double crossovers happened compared to what was expected based on single crossover frequencies. A coefficient of 1 means that observed and expected double crossovers are the same, indicating no interference. However, variations from 1 show interference level, informing geneticists about crossover patterns between genes.

Genetic Interference

Genetic interference refers to the phenomenon where the occurrence of a crossover event in one region of a chromosome affects the likelihood of another crossover occurring nearby. It is assessed using the coefficient of coincidence.

If the coefficient of coincidence is less than 1, it signifies positive interference. This means that a crossover reduces the likelihood of another one nearby, suggesting areas where crossing over is less random or frequent. Conversely, if the coefficient is greater than 1, negative interference is present, indicating crossovers happen independently, often allowing multiple events in close proximity without one affecting the other.

This insight is crucial for genetic studies as it impacts how accurately genetic maps can be constructed and interpreted, helping scientists anticipate the complexities of inheritance and gene association patterns in organisms like Drosophila.