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?

Key Concepts

Gene Mapping and Linkage

Gene mapping involves determining the location of genes on a chromosome and understanding their relative distances, which can indicate how likely they are to be inherited together. In this case, the fruitless (fru) and Antennapedia (Ant) genes are 15 centimorgans (cM) apart, suggesting they are linked but can still assort independently during gamete formation. This linkage affects the expected phenotypic ratios in offspring.

Dominant and Recessive Alleles

Alleles are different forms of a gene, and they can be classified as dominant or recessive based on their expression in the phenotype. A dominant allele, such as the newly induced Antᵈ, will mask the effect of a recessive allele when present. Understanding how these alleles interact is crucial for predicting the phenotypes of the offspring in genetic crosses.

Punnett Squares and Phenotypic Ratios

Punnett squares are a tool used to predict the genotypic and phenotypic ratios of offspring from genetic crosses. By analyzing the combinations of alleles from the parents, one can determine the expected proportions of different phenotypes in the F₂ generation. This method is essential for interpreting the results of the cross between the dominant Antᵈ mutant and the recessive fru mutant.