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.

Verified step by step guidance

1. Understand the genetic basis of the Antennapedia (Ant) gene: The Antennapedia gene is a homeotic gene that plays a critical role in determining the identity of body segments in Drosophila. Mutations in this gene can lead to transformations of one body part into another, such as legs developing in place of antennae.
2. Explain the dominance of the Antᵈ mutation: Dominant mutations often result from a gain-of-function mutation, where the altered gene product acquires a new or enhanced function. In this case, the Antᵈ mutation likely causes ectopic expression of the Antennapedia gene in the head region, leading to the development of legs instead of antennae.
3. Describe the molecular mechanism of the Antᵈ mutation: The Antᵈ mutation may involve a regulatory mutation in the Antennapedia gene, such as a change in its promoter or enhancer region. This could lead to inappropriate activation of the gene in tissues where it is not normally expressed, such as the head.
4. Relate the phenotype to the mutation: The novel phenotype of legs replacing antennae is a direct result of the misexpression of the Antennapedia gene in the head region. This misexpression overrides the normal developmental program for antennae and instead activates the developmental program for legs.
5. Summarize the explanation: The Antᵈ mutation is dominant because it causes a gain-of-function effect, leading to ectopic expression of the Antennapedia gene in the head. This misexpression results in the transformation of antennae into legs, explaining the novel phenotype observed in the mutant flies.

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, such as 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 changes, 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 phenotypes. In the case of the Antᵈ mutant, the dominant allele likely produces a functional protein that alters the developmental pathway, resulting in legs forming in place of antennae, showcasing how gene function directly correlates with phenotype.

Chromosomal Mapping and Genetic Distance

Chromosomal mapping involves determining the location of genes on a chromosome and their relative distances, measured in centimorgans (cM). The fact that the fru and Ant genes are 15 cM apart suggests that they may assort independently during gamete formation. This distance can influence the likelihood of recombination events occurring between the two genes, which is important for understanding inheritance patterns and the expression of traits in offspring.