Problem 12 - Campbell Biology

Question

Synthesis/Evaluation SCIENTIFIC INQUIRY \(\bullet\) DRAW IT Assume you are mapping genes \(A, B, C,\) and \(D\) in Drosophila. You know that these genes are linked on the same chromosome, and you determine the recombination frequencies between each pair of genes to be as follows: \(A-B, 8 \% ; A-C, 28 \% ; A-D, 25 \% ; B-C, 20 \% ; B-D, 33 \% .\) (a) Describe how you determined the recombination frequency for each pair of genes. (b) Draw a chromosome map based on your data.

Step-by-Step Solution

Answer

Start with A as reference. B is 8% away, C is 28% on the other side. Position D at correct frequencies from A, B, and C.

Step 1: Understanding Recombination Frequencies

Recombination frequency between two genes is the percentage of offspring in which a crossover event occurs between those two genes. This value indicates how far apart the genes are on the chromosome.

Step 2: List the Recombination Frequencies Given

Refer to the given recombination frequencies:y between:a) A and B: 8%b) A and C: 28%c) A and D: 25%d) B and C: 20%e) B and D: 33%

Step 3: Determine the Gene Order

Using the recombination frequencies:- Start with gene A as a reference.- Gene B is placed 8% from A because A-B frequency is 8%.- Gene C has the greatest distance from A (28%), while B-C has 20% recombination frequency. This places C further from A and B closer to A.- Gene D's position should be examined using A-D and B-D frequencies. Since B-D recombination is highest (33%), it suggests D is past C from B side.

Step 4: Construct the Chromosome Map

From the calculations and understanding of distances:a) Write down:1. A2. B, 8% away3. Place C at correct frequencies from A and BFinally, position D correctly in relation to all others.genetic map (linear): A--8%--B--20%--C--5%--D.

Key Concepts

Recombination Frequency

When studying genetics, it's important to look at recombination frequency. This phrase refers to the likelihood of crossover events happening between two genes during meiosis. Indeed, recombination frequency provides an estimate of the distance between genes on the same chromosome. The greater the frequency, the further apart the genes are likely to be. To calculate recombination frequency, scientists use the formula:\[ \text{Recombination Frequency (\%)} = \left( \frac{\text{Number of recombinants}}{\text{Total number of offspring}} \right) \times 100 \]It is important to understand that recombination frequency never exceeds 50%. Frequencies beyond 50% suggest independent assortment, which means the genes are not linked.Recombination frequency helps track inheritance patterns and plays a pivotal role in constructing chromosome maps.

Chromosome Map

A chromosome map, also known as a genetic map, charts the position of genes on a chromosome based on recombination frequencies. Simply put, it illustrates how genes are ordered and spaced. Chromosome maps are essential for understanding the genetic architecture of organisms, like Drosophila, in this example.To create a chromosome map:

• Start with a reference gene
• Place other genes relative to the reference using recombination data

For instance, given the recombination frequencies:

• A-B: 8%
• A-C: 28%
• A-D: 25%
• B-C: 20%
• B-D: 33%

We establish that gene B is closest to A, followed by C and D. A typical chromosome map might look like: A--8%--B--20%--C--5%--D. This kind of visualization helps in understanding linkage and predicting inheritance patterns.

Genetic Linkage

Genetic linkage refers to the tendency of genes located on the same chromosome to be inherited together. Linked genes do not assort independently like genes on different chromosomes. The probability of them being separated during meiosis depends on their distance from each other.In the context of Drosophila, genetic linkage helps in determining the recombination frequency and gene order for genes A, B, C, and D. Genes closer to each other have a lower recombination frequency, indicating strong genetic linkage. For example, the recombination frequency between A and B is 8%, suggesting they are closely linked.In contrast, a 28% frequency between A and C implies that they are farther apart. Understanding genetic linkage is crucial for tasks like gene mapping and detecting crossover events.

Crossover Events

During meiosis, homologous chromosomes exchange genetic material in a process known as crossover. Crossover events are significant because they result in genetic recombination and contribute to genetic diversity.The occurrence of crossover events between linked genes is what produces recombinants – offspring with combinations of traits different from those in the parents.Tracking crossover events for genes like A, B, C, and D in Drosophila helps scientists map these genes on chromosomes. The frequency of crossover events between genes is directly linked to the recombination frequency, with more frequent crossovers indicating greater distances between genes.Crossover events are indispensable for creating accurate chromosome maps and understanding how traits are passed down and varied within populations.