Problem 12 - Campbell Biology

Question

Assume you are mapping genes \(A, B, C,\) and \(D\) in \(D r o s o p h i l a .\) 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: \(A\) and \(B, 8 \% ; A\) and \(C, 28 \% ; A\) and \(D, 25 \% ; B\) and \(C\) \(20 \% ; B\) and \(D, 33 \%\) (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

Determine gene order from smallest recombination frequency. Draw map placing genes to match all given recombination percentages.

Step 1: Understand Recombination Frequency

Recombination frequency reflects the likelihood of a crossover event occurring between two genes during meiosis. It is expressed as a percentage representing the proportion of offspring showing recombination between the two genes.

Step 2: Interpreting Given Data

The provided recombination frequencies are as follows: - Between A and B: 8% - Between A and C: 28% - Between A and D: 25% - Between B and C: 20% - Between B and D: 33%

Step 3: Determine Gene Order

To establish the relative position of the genes on the chromosome, begin by identifying the smallest recombination frequency. This will indicate the closest two genes. Here, A and B have the smallest recombination frequency (8%). Place A and B near each other on the chromosome map.

Step 4: Compare Distances to Place Remaining Genes

Next, identify other gene pairs relative to A and B:- A and C: 28%- A and D: 25%- B and C: 20%- B and D: 33%Since C is 28% from A and 20% from B, place C 20 units from B and 28 units from A. Similarly, D's position can be determined based on its distances from A and B.

Step 5: Construct the Chromosome Map

Using the determined distances, the chromosome map can be drawn as follows:- A ---8%--- B ---20%--- C ---5%--- D Since A to D is 25% and B to D is 33%, D should be placed after C by 5 units (since 20% + 5% = 25%). Ensure all calculated distances match the provided recombination frequencies.

Key Concepts

recombination frequency

Recombination frequency is a fundamental concept when it comes to understanding gene mapping. It reflects the likelihood that a crossover event will happen between two genes during the process of meiosis. This frequency is essentially the percentage of offspring that show new combinations of traits differing from their parents.

Let's break it down:

• When genes are closely located on a chromosome, there's a lower chance for a crossover, resulting in a smaller recombination frequency.
• Conversely, the farther apart two genes are, the higher the probability of crossing over, and thus a higher recombination frequency.
• These values are used to calculate the physical distance between genes on a chromosome, aiding in the creation of a chromosome map.

For example, if the recombination frequency between genes A and B is 8%, it means genes A and B are close to each other on the chromosome. If the frequency is 33% between B and D, they are farther apart.

chromosome map

A chromosome map visually represents the positions and distances between genes on a chromosome. Here's how you construct one using recombination frequencies:

• Start by identifying the pair of genes with the smallest recombination frequency. These genes are closest together on the chromosome. In our example, genes A and B have the smallest recombination frequency of 8%.
• Place these genes next to each other and use the other given recombination frequencies to map out the remaining genes.
• For instance, if A and C have a frequency of 28%, place gene C 28 units away from A.
• Likewise, for B and C with a 20% frequency, place C such that it is 20 units away from B.

Now, consider gene D, which showed 25% recombination frequency with A and 33% with B. Place gene D accordingly to ensure all calculated distances match the given frequencies. The chromosome map would then look like this: A --- (8%) --- B --- (20%) --- C --- (5%) --- D (since 20% + 5% = 25%). This map is a simplified representation to make sense of the gene locations relative to each other.

meiosis

Meiosis is a special type of cell division crucial for sexual reproduction. When an organism produces gametes (sperm or egg cells), meiosis ensures these cells have half the number of chromosomes compared to the parent cell. Here’s a healthier look into meiosis and its role in recombination:

• Meiosis consists of two rounds of cell division, known as meiosis I and meiosis II. During meiosis I, homologous chromosomes are separated. Each chromosome still consists of two sister chromatids.
• Before the first division, homologous chromosomes exchange genetic material in a process called recombination or crossing over.
• This exchange occurs at points called chiasmata and greatly increases genetic diversity by shuffling genes between chromosomes.

The outcome of recombination is what recombination frequency measures. By knowing recombination frequencies between genes, you can create a map of their locations on a chromosome. Understanding meiosis deepens your grasp of how genetic variation is created and why gene mapping is possible.