In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.

A wild-type female fruit fly with the genotype y⁺w⁺f/ywf⁺ is crossed to a male fruit fly that has yellow body, white eye, and forked bristles. Predict the frequency of each progeny phenotype class produced by this mating.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This is important in understanding the inheritance patterns of traits, as linked genes do not assort independently. In the context of Drosophila, the proximity of the body color, eye color, and bristle form genes on the X chromosome affects the expected phenotypic ratios in the offspring.

Map Units and Recombination Frequency

Map units, or centimorgans (cM), are a measure of genetic distance based on the frequency of recombination between two loci during meiosis. One map unit corresponds to a 1% chance of recombination occurring between two genes. In the given question, the distances between the genes on the X chromosome (0.0, 1.5, and 56.7 m.u.) will help predict the phenotypic ratios of the progeny by indicating how often recombination will separate the alleles.

Dominance and Allelic Interactions

Dominance describes the relationship between alleles of a gene, where the dominant allele masks the expression of the recessive allele in a heterozygous individual. In this scenario, the wild-type alleles (y⁺, w⁺, f⁺) are dominant over their respective mutant alleles (y, w, f). Understanding this concept is crucial for predicting the phenotypes of the progeny, as the presence of dominant alleles will determine the observable traits in the offspring.