In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F₁ progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table, Phenotype Number Dichaete 401 ebony, pink 389 Dichaete, ebony 84 pink 96 Dichaete, pink 2 ebony 3 Dichaete, ebony, pink 12 wild type 13 Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles can be classified as dominant or recessive based on their phenotypic expression. A dominant allele, such as the Dichaete mutation (D), will manifest in the phenotype even if only one copy is present. In contrast, recessive alleles, like ebony (e) and pink (p), require two copies to express their traits. Understanding this distinction is crucial for predicting the outcomes of genetic crosses.

Homozygous and Heterozygous Genotypes

An organism is homozygous for a gene when it has two identical alleles, while it is heterozygous when it has two different alleles. In this scenario, the Dichaete mutation is lethal when homozygous, meaning that individuals with two copies of the D allele do not survive. Recognizing these genotypic classifications helps in analyzing the inheritance patterns observed in the offspring of the crosses.

Backcrossing and Phenotypic Ratios

Backcrossing involves mating an F₁ hybrid organism with one of its parental genotypes to analyze inheritance patterns. The resulting offspring's phenotypic ratios provide insights into the genetic contributions of the parents. In this case, the backcross of Dichaete F₁ flies to homozygous ebony, pink flies reveals the segregation of traits and helps determine the linkage and interaction between the genes involved.