The locations of six deletions have been mapped to the...

The locations of six deletions have been mapped to the Drosophila chromosome, as shown in the following deletion map. Recessive mutations $a, b, c, d, e,$ and $f$ are known to be located in the same region as the deletions, but the order of the mutations on the chromosome is not known. (FIGURE CANNOT COPY) When flies homozygous for the recessive mutations are crossed with flies homozygous for the deletions, the following results are obtained, in which "m" represents a mutant phenotype and a plus sign ( $+)$ represents the wild type. On the basis of these data, determine the relative order of the seven mutant genes on the chromosome: (TABLE CANNOT COPY)

Key Concepts

Recessive Mutation Analysis

Recessive mutation analysis focuses on mutations that only exhibit a mutant phenotype when present in two copies (homozygous state). In mapping studies, crossing organisms that are homozygous for recessive mutations with those carrying deletions helps reveal allelic relationships and the chromosomal locations of genes, as the mutant phenotype will appear if the single wild-type allele provided by the deletion does not complement the recessive mutation.

Deletion Mapping

Deletion mapping is a genetic technique in which known segments of a chromosome are intentionally removed (deleted) to determine the chromosomal locations of genes. By analyzing the phenotypes of organisms that carry these deletions when combined with other mutations, researchers can identify which genes lie within the deleted regions.

Complementation Analysis

Complementation analysis is used to determine whether two mutations affect the same gene or different genes. When two mutations are in different genes, each allele can supply the function missing in the other, resulting in a wild-type phenotype. In the context of deletion mapping, the failure to complement (resulting in a mutant phenotype) indicates that the gene affected by the mutation is located in the region of the deletion.

Gene Order Determination

Gene order determination involves arranging genes sequentially along a chromosome based on genetic interactions and mapping data. By examining which deletions fail to complement specific mutations, one can infer the relative positions of the genes. This method allows for the construction of a linear gene map, showing the order of genes along the chromosome.

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