The boss in your laboratory has just heard of a proposal by another laboratory that genes for eye color and the length of body bristles may be linked in Drosophila. Your lab has numerous pure-breeding stocks of Drosophila that could be used to verify or refute genetic linkage. In Drosophila, red eyes (c⁺) are dominant to brown eyes (c) and long bristles (d⁺) are dominant to short bristles (d). Your lab boss asks you to design an experiment to test the genetic linkage of eye color and bristle-length genes, and to begin by crossing a pure-breeding line homozygous for red eyes and short bristles to a pure-breeding line that has brown eyes and long bristles.


Assume the eye color and bristle-length genes are separated by 28 m.u. What are the approximate frequencies of phenotypes expected from the cross you proposed in part (b)?

Verified step by step guidance

1. Step 1: Begin by understanding the genetic setup. The genes for eye color (c⁺/c) and bristle length (d⁺/d) are being tested for linkage. Red eyes (c⁺) are dominant to brown eyes (c), and long bristles (d⁺) are dominant to short bristles (d). The parental generation consists of two pure-breeding lines: one homozygous for red eyes and short bristles (c⁺c⁺dd) and the other homozygous for brown eyes and long bristles (ccd⁺d⁺).
2. Step 2: Perform the initial cross between the two pure-breeding lines. The F₁ generation will inherit one allele from each parent. Since the parental lines are homozygous, all F₁ offspring will be heterozygous for both traits (c⁺c d⁺d). This generation will exhibit the dominant phenotypes: red eyes and long bristles.
3. Step 3: To test for genetic linkage, perform a test cross. Cross the F₁ heterozygous individuals (c⁺c d⁺d) with a homozygous recessive individual (cc dd). This will allow you to observe the recombination frequencies between the two genes.
4. Step 4: Use the given genetic distance of 28 m.u. (map units) between the eye color and bristle-length genes to predict recombination frequencies. Recall that 1 m.u. corresponds to a 1% recombination frequency. Therefore, 28 m.u. implies a 28% chance of recombination between the two genes. This means that 72% of the offspring will inherit parental combinations of alleles (c⁺d⁺ and cd), while 28% will inherit recombinant combinations (c⁺d and cd⁺).
5. Step 5: Calculate the expected phenotypic frequencies based on the recombination data. Parental phenotypes (red eyes with long bristles and brown eyes with short bristles) will make up 72% of the offspring, while recombinant phenotypes (red eyes with short bristles and brown eyes with long bristles) will make up 28%. Divide these percentages equally among the four phenotypes to determine the approximate frequencies for each.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. This phenomenon can affect the expected ratios of offspring phenotypes in genetic crosses. When genes are linked, the recombination frequency between them is lower than expected, which can be quantified in map units (m.u.), indicating the distance between genes.

Drosophila Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a model organism in genetics due to its simple genetic structure and short life cycle. In this context, traits such as eye color and bristle length are determined by specific alleles, with red eyes (c⁺) and long bristles (d⁺) being dominant. Understanding the inheritance patterns of these traits is crucial for predicting the outcomes of genetic crosses.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different observable traits in the offspring resulting from a genetic cross. In this experiment, the expected phenotypic ratios can be calculated based on the principles of Mendelian inheritance and the degree of genetic linkage. By analyzing the expected ratios, researchers can determine whether the genes for eye color and bristle length are linked or assort independently.