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: Provide a molecular explanation for the reason your new Antᵈ mutant is dominant and for its novel phenotype.

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles can be classified as dominant or recessive based on their expression in the phenotype. A dominant allele, like the newly induced Antᵈ mutant, will mask the effect of a recessive allele when both are present in a heterozygous individual. This means that even if a recessive allele is present, the dominant trait will be expressed, leading to observable phenotypic differences, such as the development of legs instead of antennae.

Gene Function and Phenotype

Genes encode proteins that perform specific functions in an organism, influencing its development and traits. The Antennapedia gene is crucial for proper limb development, and mutations can lead to abnormal structures. The novel phenotype of the Antᵈ mutant, which causes legs to develop in place of antennae, suggests a gain-of-function mutation that alters the normal expression pattern of the gene, resulting in the misplacement of body structures.

Genetic Mapping and Linkage

Genetic mapping involves determining the location of genes on chromosomes and understanding their distances from one another, often measured in centimorgans (cM). The fact that the fru and Ant genes are 15 cM apart indicates they are linked but can assort independently during gamete formation. This linkage can affect inheritance patterns, especially when considering the dominance of the Antᵈ allele in the context of a cross with a fru mutant, influencing the resulting phenotypes in the offspring.