Problem 9 - Campbell Biology Concepts & Connections

Question

A fruit fly with a gray body and red eyes (genotype \(B b P p\) ) is mated with a fly having a black body and purple eyes (genotype \(b b p p\) ). What ratio of offspring would you expect if the body-color and eye-color genes are on different chromosomes (unlinked)? When this mating is actually carried out, most of the offspring look like the parents, but \(3 \%\) have a gray body and purple eyes, and \(3 \%\) have a black body and red eyes. Are these genes linked or unlinked? What is the recombination frequency?

Step-by-Step Solution

Answer

Genes are linked with a recombination frequency of 6%.

Step 1: Define the Genotypes and Phenotypes

Identify the genotypes and phenotypes for both flies. The first fly has genotype \(B b\) for body color and \(P p\) for eye color, indicating gray body (dominant B) and red eyes (dominant P). The second fly has genotype \(b b\) for body color and \(p p\) for eye color, indicating black body (recessive b) and purple eyes (recessive p).

Step 2: Make a Punnett Square

Draw a Punnett square for the cross between \(B b\) and \(b b\) for body color, and another Punnett square for the cross between \(P p\) and \(p p\) for eye color. Combine these squares to determine the expected ratios of offspring if the genes are unlinked. The combined Punnett square will have genotype combinations: \(B b P p\), \(B b p p\), \(b b P p\), and \(b b p p\).

Step 3: Determine Expected Ratios

From the Punnett square, calculate the expected ratios. Since the genes are unlinked, each gene segregates independently. Therefore, the phenotype ratios should be: 1 gray body/red eyes : 1 gray body/purple eyes : 1 black body/red eyes : 1 black body/purple eyes. This translates to a 1:1:1:1 ratio.

Step 4: Analyze the Actual Observed Ratios

In the given scenario, most offspring resemble the parents, with 3% having gray body/purple eyes and 3% with black body/red eyes. These are the recombinant types, totaling 6% of the offspring.

Step 5: Calculate Recombination Frequency

Recombination frequency is calculated as the total number of recombinant offspring divided by the total number of offspring. Here, the recombination frequency is given by \(\frac{3\text{% Gray body/Purple eyes} + 3\text{% Black body/Red eyes}}{100\text{% total}} = 6\text{%}\). This implies that the recombination frequency is 6%.

Step 6: Determine Linkage

Typically, if the recombination frequency is less than 50%, the genes are linked. In this case, the recombination frequency is 6%, which suggests that the body color and eye color genes are linked.

Key Concepts

Punnett square

A Punnett square is a handy diagram used to predict the possible genotypes of offspring from a particular cross or breeding experiment. This tool visualizes how alleles from each parent combine. In this exercise, we use the Punnett square to track two traits: body color and eye color in fruit flies. The first fly has the genotype \(Bb\) for body color (gray) and \(Pp\) for eye color (red), while the second fly has the genotype \(bb\) for body color (black) and \(pp\) for eye color (purple). By crossing each possibility (i.e., each parent's alleles), we can predict the offspring's genotypes:

• \(Bb Pp\)
• \(Bb pp\)
• \(bb Pp\)
• \(bb pp\)

The expected ratio is derived by combining each outcome, assuming genes are unlinked.

Recombination frequency

Recombination frequency helps to measure the likelihood that two genes will cross over during meiosis, the process of forming gametes (sex cells). It's calculated by comparing the observed number of recombinant offspring to the total number of offspring. In our case, we observed recombinant offspring with gray body/purple eyes and black body/red eyes, each accounting for 3% of the population. The total recombination frequency is therefore \[ \frac{3\text{%} + 3\text{%}}{100\text{%}} = 6\text{%} \]. This frequency is crucial for determining if genes are linked or independently assorted.

Gene segregation

Gene segregation refers to how genes separate during gamete formation in meiosis. According to Mendel's Law of Segregation, each parent passes one allele for each trait to their offspring. When dealing with two traits, such as body color and eye color, each gene should segregate independently if the genes are unlinked. This scenario follows Mendel's Law of Independent Assortment, which we analyzed using Punnett squares for each individual trait and then combined them. The expected 1:1:1:1 ratio assumes independent assortment. However, observed deviations from these ratios suggest linkage.

Linked genes

Linked genes are genes located close to each other on the same chromosome. These genes tend to be inherited together because they are less likely to be separated during recombination in meiosis. In the fruit fly exercise, the observed 6% recombination frequency (well below 50%) strongly indicates that the body color and eye color genes are linked. Most offspring resembled their parents, aligning with the idea that crossing over between these genes is infrequent, thereby maintaining the parental combination more often than would be expected from unlinked genes.

Phenotype ratios

Phenotype ratios describe the relative number of offspring with different physical trait combinations resulting from a particular genetic cross. In this example, the expected phenotype ratio for unlinked genes would be 1:1:1:1, including all combinations like gray body/red eyes and black body/purple eyes. However, actual results showed most offspring looked like the parents, with only 3% gray body/purple eyes and 3% black body/red eyes. These deviations indicate linked genes leading to fewer recombinants than expected, thus altering the phenotype ratios from the 1:1:1:1 assumption for unlinked traits.