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.

In a cross involving these three X-linked genes, do you expect any gene pair(s) to show genetic linkage? Explain your reasoning.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. This occurs because linked genes are less likely to be separated by recombination events. The closer the genes are on the chromosome, the stronger the linkage, which can be measured in map units (m.u.). In the context of the question, analyzing the distances between the genes helps determine if they are likely to be linked.

Map Units (m.u.)

Map units, or centimorgans (cM), are a unit of measurement for genetic linkage that reflects the distance between genes on a chromosome. One map unit corresponds to a 1% chance of recombination occurring between two genes during meiosis. In the provided scenario, the distances between the genes (0.0 to 1.5 m.u. for body color and eye color, and 1.5 to 56.7 m.u. for eye color and bristle form) indicate the likelihood of linkage, with smaller distances suggesting stronger linkage.

Dominance and Alleles

Dominance in genetics refers to the relationship between alleles, where one allele (the dominant) masks the expression of another (the recessive) in a heterozygous individual. In the question, the wild-type alleles (y⁺, w⁺, f⁺) are dominant over their respective mutant alleles (y, w, f). Understanding dominance is crucial for predicting phenotypic outcomes in crosses, especially when considering how linked genes may affect inheritance patterns.