In Drosophila, seven partial deletions (1 to 7) shown as gaps in the following diagram have been mapped on a chromosome. This region of the chromosome contains genes that express seven recessive mutant phenotypes, identified in the following table as a through g. A researcher wants to determine the location and order of genes on the chromosome, so he sets up a series of crosses in which flies homozygous for a mutant allele are crossed with flies homozygous for a partial deletion. The progeny are scored to determine whether they have the mutant phenotype ('m' in the table) or the wild-type phenotype ('+' in the table). Use the partial deletion map and the table of progeny phenotypes to determine the order of genes on the chromosome.

Verified step by step guidance

1. Step 1: Understand the problem. The goal is to determine the order of genes (a through g) on a chromosome based on the results of crosses between flies homozygous for mutant alleles and flies homozygous for partial deletions. The progeny phenotypes ('m' for mutant and '+' for wild-type) indicate whether a gene is located within a specific deletion region.
2. Step 2: Analyze the deletion map. The deletion map shows seven partial deletions (1 to 7) as gaps on the chromosome. Each deletion removes a specific segment of the chromosome, and the genes located within that segment will not be expressed in the progeny if the deletion overlaps with the gene's location.
3. Step 3: Examine the progeny phenotype table. For each deletion (1 to 7), the table indicates whether the progeny exhibit the mutant phenotype ('m') or the wild-type phenotype ('+'). If the progeny show the mutant phenotype ('m'), it means the gene is located within the deleted region. If the progeny show the wild-type phenotype ('+'), the gene is not within the deleted region.
4. Step 4: Determine the location of each gene. For each gene (a through g), compare its phenotype across the seven deletions. Identify the deletions where the progeny show the mutant phenotype ('m'). The gene must be located within the overlapping region of those deletions.
5. Step 5: Establish the order of genes. Once the location of each gene is determined, arrange the genes in order based on their positions along the chromosome. Use the deletion map as a reference to ensure the order aligns with the physical layout of the chromosome.

Key Concepts

Chromosomal Mapping

Chromosomal mapping is a technique used to determine the relative positions of genes on a chromosome. It involves analyzing the inheritance patterns of traits in offspring to infer the locations of genes based on recombination frequencies. In this context, the researcher uses partial deletions to identify which genes are present or absent in the progeny, helping to establish their order on the chromosome.

Recessive Mutant Phenotypes

Recessive mutant phenotypes are traits that are expressed only when an organism has two copies of the mutant allele. In Drosophila, these phenotypes can be identified by crossing flies with known mutations and observing the resulting offspring. Understanding which phenotypes correspond to specific deletions allows researchers to deduce the arrangement of genes based on the presence or absence of these traits in the progeny.

Genetic Crosses

Genetic crosses involve mating individuals with specific genotypes to study the inheritance of traits in their offspring. In this scenario, the researcher crosses homozygous mutant flies with those carrying partial deletions to analyze the resulting phenotypes. By scoring the progeny for mutant and wild-type traits, the researcher can infer the linkage and order of the genes associated with the observed phenotypes.