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.

Explain how each of the predicted progeny classes is produced.

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 due to their physical proximity, which reduces the likelihood of recombination occurring between them. In the context of Drosophila, the genes for body color, eye color, and bristle form are linked, affecting the expected ratios of progeny phenotypes based on their map distances.

Map Units and Recombination Frequency

Map units, or centimorgans (cM), are a measure of genetic distance based on the frequency of recombination between loci during meiosis. One map unit corresponds to a 1% chance of recombination occurring between two genes. In the given Drosophila example, the distances between the body color and eye color genes (1.5 m.u.) and the bristle form gene (56.7 m.u.) indicate how likely offspring will inherit combinations of these traits.

Dominance and Phenotypic Expression

Dominance describes the relationship between alleles, where the presence of a dominant allele masks the expression of a recessive allele in the phenotype. 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 phenotypic ratios of the progeny based on the combinations of alleles inherited from the parents.