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.

Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

Verified step by step guidance

1. Step 1: Define the alleles and their dominance relationships. Dichaete (D) is a dominant mutation on chromosome III and lethal when homozygous (D/D lethal). ebony (e) and pink (p) are recessive mutations on the same chromosome. The wild-type alleles are d (for Dichaete), E (for ebony), and P (for pink).
2. Step 2: Determine the genotypes of the parental flies in the initial cross. The Dichaete stock flies must be heterozygous for Dichaete (D/d) and carry wild-type alleles for ebony and pink (E/P), since ebony and pink mutations are recessive and not expressed in this stock. The ebony, pink flies are homozygous recessive for both ebony and pink (e/e, p/p) and wild-type for Dichaete (d/d).
3. Step 3: Write the genotype of the F₁ progeny from the initial cross. Since D is dominant and lethal when homozygous, the F₁ flies with the Dichaete phenotype are heterozygous for Dichaete (D/d) and heterozygous for ebony and pink (E/e, P/p).
4. Step 4: Diagram the backcross between the F₁ heterozygous flies (D/d, E/e, P/p) and the homozygous recessive ebony, pink flies (d/d, e/e, p/p). List all possible gametes from each parent considering the three loci and then combine them to predict offspring genotypes.
5. Step 5: Use the phenotypic data from the backcross table to assign phenotypes to genotypes. For example, flies with Dichaete phenotype carry at least one D allele, ebony phenotype requires homozygous e/e, and pink phenotype requires homozygous p/p. This will help in confirming the parental genotypes and the arrangement of alleles on the chromosomes.

Key Concepts

Linkage and Recombination

Linkage refers to genes located close together on the same chromosome that tend to be inherited together. Recombination occurs during meiosis when crossing over between homologous chromosomes creates new allele combinations. The frequency of recombinant offspring helps map gene distances and order on chromosomes.

Dominant and Recessive Mutations

Dominant mutations express their phenotype when present in one copy, while recessive mutations require two copies to show their effect. In this question, Dichaete is dominant and lethal when homozygous, whereas ebony and pink are recessive mutations, influencing how genotypes translate into phenotypes in crosses.

Backcross and Genetic Cross Diagrams

A backcross involves crossing a hybrid organism with one of its parents or a genetically similar individual to analyze inheritance patterns. Diagramming crosses with genotypes and phenotypes clarifies allele segregation, linkage, and recombination events, essential for interpreting offspring ratios and mapping genes.