In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males, and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below:
What phenotypes, and in what proportions, do you expect in the F₂ obtained by interbreeding F₁ animals?

Verified step by step guidance

1. Step 1: Understand the genetic setup. The problem involves two genes: the fruitless gene (fru) and the Antennapedia gene (Ant). The fru gene has a recessive mutation (fru⁻), and the Ant gene has a newly induced dominant mutation (Antᵈ). The two genes are located 15 cM apart on chromosome 3, meaning there is a 15% chance of recombination between them.
2. Step 2: Analyze the parental cross. The pure-breeding Antᵈ mutant (Antᵈ/Antᵈ, fru⁺/fru⁺) is crossed with a homozygous recessive fru mutant (Ant⁺/Ant⁺, fru⁻/fru⁻). The F₁ offspring will inherit one allele from each parent, resulting in the genotype Antᵈ/Ant⁺, fru⁺/fru⁻ for all F₁ individuals.
3. Step 3: Determine the F₁ intercross. The F₁ individuals (Antᵈ/Ant⁺, fru⁺/fru⁻) are interbred to produce the F₂ generation. Since the two genes are linked, we need to account for both independent assortment and recombination between the loci. Use a Punnett square or branching diagram to track the inheritance of the alleles.
4. Step 4: Calculate the expected phenotypes. The phenotypes in the F₂ generation will depend on the combinations of alleles inherited. The Antᵈ allele is dominant and causes legs to develop instead of antennae, while the fru⁻ allele is recessive and causes males to court other males. Consider the recombination frequency (15%) to determine the proportion of recombinant genotypes versus parental genotypes.
5. Step 5: Assign phenotypes to genotypes and calculate proportions. Based on the genotypes, assign the corresponding phenotypes (e.g., wild type, Antᵈ mutant, fru⁻ mutant, or double mutant). Combine the probabilities of parental and recombinant genotypes to determine the expected proportions of each phenotype in the F₂ generation.

Key Concepts

Gene Interaction and Epistasis

Gene interaction refers to the way different genes influence each other's expression and the resulting phenotype. In this case, the interaction between the fruitless (fru) and Antennapedia (Ant) genes is crucial, as mutations in these genes lead to distinct phenotypic outcomes. Epistasis occurs when the effect of one gene is masked or modified by another gene, which is relevant for understanding how the dominant Antdᵈ allele may affect the expression of the fru gene in the F₂ generation.

Dominance and Recessiveness

Dominance and recessiveness are fundamental concepts in genetics that describe how alleles interact to determine phenotype. A dominant allele, like Antdᵈ, can mask the effect of a recessive allele, such as fru. Understanding these relationships is essential for predicting the phenotypic ratios in the offspring, particularly when crossing different genotypes, as seen in the F₁ and F₂ generations.

Mendelian Inheritance and Punnett Squares

Mendelian inheritance describes the patterns of inheritance that are characteristic of many traits, based on Gregor Mendel's principles. Using Punnett squares allows for the visualization of genetic crosses and the prediction of offspring genotypes and phenotypes. In this scenario, constructing a Punnett square for the F₁ generation will help determine the expected phenotypic ratios in the F₂ generation, based on the alleles contributed by the parents.