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. [Two diagrams appear here - see next page] <>

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 genes and their associated phenotypes in offspring. In this case, the researcher uses partial deletions to identify which genes are present or absent in the progeny, allowing for the construction of a gene map based on observed phenotypes.

Recessive Mutant Phenotypes

Recessive mutant phenotypes occur when an organism has two copies of a mutant allele, leading to the expression of a specific trait. In Drosophila, these phenotypes can be identified through crosses with wild-type flies. Understanding which phenotypes are recessive helps in determining the genetic basis of traits and their linkage to specific chromosomal regions.

Genetic Crosses and Progeny Analysis

Genetic crosses involve mating individuals with specific genotypes to observe the inheritance of traits in their offspring. By analyzing the phenotypes of the progeny, researchers can infer the genetic relationships between alleles and their locations on chromosomes. In this scenario, the researcher assesses the phenotypes of progeny from crosses between mutant and deletion homozygotes to deduce the order of genes on the chromosome.