A Drosophila experiment examining potential genetic linkage of X-linked genes studies a recessive eye mutant (echinus), a recessive wing-vein mutation (crossveinless), and a recessive bristle mutation (scute). The wild-type phenotypes are dominant. Trihybrid wild-type females (all have the same genotype) are crossed to hemizygous males displaying the three recessive phenotypes. Among the 20,765 progeny produced from these crosses are the phenotypes and numbers listed in the table. Any phenotype not given is wild type. Phenotype Number 1. Echinus     8576 2. Scute       977 3. Crossveinless        716 4. Echinus, scute       681 5. Scute, crossveinless     8808 6. Scute, crossveinless, echinus         4 7. Echinus, crossveinless     1002 8. Wild type         1 20,765 Determine the gene order and identify the alleles on the homologous X chromosomes in the trihybrid females.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This phenomenon occurs because linked genes are less likely to be separated by recombination events. Understanding genetic linkage is crucial for analyzing inheritance patterns, especially in experiments involving multiple traits, as it helps in determining the relative positions of genes on a chromosome.

Recessive and Dominant Alleles

In genetics, alleles can be classified as dominant or recessive based on their phenotypic expression. Dominant alleles mask the effect of recessive alleles in heterozygous individuals, leading to the expression of the dominant trait. In the context of the Drosophila experiment, the wild-type phenotypes are dominant, while the mutations studied are recessive, which is essential for understanding the inheritance patterns observed in the progeny.

Phenotypic Ratios and Chi-Square Analysis

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring of a genetic cross. Analyzing these ratios helps in determining the genetic relationships and potential linkage between traits. Chi-square analysis can be employed to assess whether the observed phenotypic ratios significantly deviate from expected ratios, providing insights into gene order and linkage relationships among the alleles in the experiment.