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)?

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 occurs because linked genes do not assort independently, leading to a higher probability of certain combinations of traits appearing together in offspring. Understanding linkage is crucial for predicting the outcomes of genetic crosses, especially when calculating phenotype frequencies.

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 Drosophila, traits such as eye color and bristle length are determined by specific alleles, with dominance relationships influencing the phenotypic expression. Familiarity with Drosophila genetics allows researchers to design experiments and interpret results effectively, particularly in studies of inheritance patterns.

Map Units and Recombination Frequency

Map units (m.u.) are a measure of genetic distance between genes, where 1 m.u. corresponds to a 1% chance of recombination occurring between two genes during meiosis. In the context of the question, a distance of 28 m.u. indicates that there is a 28% chance of recombination between the eye color and bristle-length genes. This information is essential for predicting the expected phenotype ratios in the offspring of the genetic cross.